The Heroes of Olympus/WMG

One of the The Kanes will be the seventh hero
The team, as of now, breaks down into with an extra from one of the camps wrecking the balance. However if it were to go 3 Romans, 3 Greeks and 1 Other, there would be a sort of balance. Also, I'm a sucker for that type of crossover junk.
 * Greek: Percy, Leo, Piper
 * Roman: Jason, Reyna, ???
 * Given that Aphrodite told Piper that she would be the mediator who brings about cooperation between the two camps, it would seem to be 3 Greeks (Percy, Annabeth, and Leo) 3 Romans (Jason, Reyna?, and uknown) and 1 other-- Piper, who brings the two sides together.
 * Or, we could simply see it as 4 Greeks and 3 Romans. The Greek campers get one more because they've held the helm of the series from the beginning, the greek version of the myths are the most well-known (most of the time, never mind Hercules) and we're used to them by now. Agreed that the greeks would be Piper, Leo (already stated as pretty much a fact), Percy (obviously), and the fourth is likely to be Annabeth because unless something horrible happens to her (see other WMG theories in this page), she's not going to just wait while Percy goes off to the Doors of Death; she's his girlfriend and has been his lancer all through the first series. From the roman camp we have Jason, I'm betting Reyna (especially if there's a Love Dodecahedron situation involved), and someone else.
 * Seeing as Aphrodite was the mother of the first Roman demigod, and Piper is slated to be the mediator between the two sides, she'll most likely be seen as neutral
 * You're all wrong - three of the heroes will be Leucos, Axios, and Eris.
 * Riordan has confirmed that the seven demigods are children of Zeus, Poseidon, Hades, Hephaestus, Athena, Ares, and Aphrodite.
 * All right, so that's Jason, Percy, Nico, Leo, Annabeth, Clarisse, and Piper, respectively.
 * It's very unlikely to be Clarisse. Given how much more the god of war is respected as Mars than Ares, his demigod will almost definitely be a Roman.
 * Plus, there's a good chance that the child of Hades would be from the Roman camp (as a child of Pluto). Whilst Nico is powerful in his own right, to only have two people from the Roman camp (i.e. Jason and this child of Mars) as part of the seven would be really unfair. As stated above, 3 from the Greek Camp and 3 from the Roman Camp with Piper as the middle ground seems the most likely.
 * is the Ares kid.
 * But that would mean that, after the big deal they made of Hades being the only one to stay true to the oath, he would've secretly broken it anyway. From what we've seen of him, he doesn't seem the type. It'd make much more sense if the extra Roman was a child of Minerva. Annabeth doesn't have to be in the prophecy to be involved in the war.
 * Thats if the oath covers all of their aspects. We know that Zeus broke it with Jason as his Jupiter aspect (though thats not saying much considering how faithful Zeus is). We know that they have completely different personalities as their Roman aspects, so it is still fairly possible that Pluto has a child at the Roman camp. This troper will admit though to leave Annabeth out of this prophecy would be a good plot twist.
 * As of now it is most likely that the 7 heroes will be Percy, Jason, Piper, Leo, Annabeth, and the two new characters that were introduced in Son of Neptune: Hazel and Frank
 * As of now it is most likely that the 7 heroes will be Percy, Jason, Piper, Leo, Annabeth, and the two new characters that were introduced in Son of Neptune: Hazel and Frank

Nico will have a Moment of Awesome and stop a load of evil dead people from coming to life.
But of course he will let the good people out. Many happy moments will occur as beloved but dead characters return for one final battle.

Annabeth will be the seventh hero
Contradicting my last WMG with what is honestly much more likely.

Annabeth dies
Arachne kills Annabeth and the seven heroes go to the Doors of Death to rescue her.

Divine parentage of the seven
Jason, Piper, and Leo are confirmed. Nico and Percy seem inevitable. That's Zeus, Poseidon, Hades, Aphrodite, and Hephaestus. On the grounds that a child of Hermes played a big role last series, the remaining two heroes (Annabeth and Reyna?) will come from Athena, Ares, Apollo, or Demeter.
 * Well, Clarisse did have her epic Big Damn Heroes scene in TLO...My money's on Apollo. Every good raid group needs a ranged character/healer.
 * My money's on a SON of Artemis. Just for Irony.
 * Not likely for obvious reasons. Artemis will have to have made him out of a flower or woodland critter or moonlight or something, is all I'm saying.
 * Sculpture made out of moon-rock is my bet. Also, I believe she loved two men in Greek Mythology. One died (Orion, I think), the other was put into an eternal sleep (Endymion). Third time's the charm.
 * One man, Orion, whom Apollo tricked Artemis into killing because he wanted to preserve her virginity. Endymion was Selene, a titan.
 * Actually, Diana (Her Roman form) fell in love with both.
 * My guess is that Apollo and Demeter will finally get some love - Apollo's had one scene and only a couple off-hand mentions in the series. Demeter had a couple scenes more than Apollo but we don't really know any of her children, same with Apollo.
 * As cool as having seven children of seven different gods form the prophesized group would be, why does Nico "seem inevitable"? Granted, the kid is an Ensemble Darkhorse of the grandest kind, but...
 * A major problem for the demigods right now is that slain monsters are regenerating almost immediately because of that tunnel Gaia made into Tartarus. But monsters killed by Stygian iron never regenerate because their essence is absorbed into the metal. And since Stygian iron can only be wielded by children of the Underworld, Nico's the only demigod who can kill monsters that will actually stay dead. So it stands to reason that he'll be a key player in all this one way or another.
 * Since Jason's one the seven, Thalia probably won't be, assuming a limit of one hero per god. Combined with everything that's going on with Gaia smuggling people out of the Underworld, the Stygian iron thing, and Hades being one of the Big Three, Nico seems like a pretty safe bet.
 * Who says it has to be Nico, though? We don't know if the Roman aspects of the gods are bound by the same oaths as their Greek forms, so there could be a child of Pluto running around at the Roman camp who could be one of the seven.
 * If the Romans received the current Great Prophecy, they probably received the last one, too. So gods are probably bound to their oaths no matter what their form. They're still the same gods, after all.
 * But they aren't, thats the point of this series. When we first learn about the aspects from Clovis, he states that they have different personalities and as a result different Gods. There is a chance that they did not recieve the previous Great Prophecy and even if they did, they broke the rules anyway in their Greek Forms and in their Roman forms so there is still a chance that there is a child of Pluto running around who is more experienced then Nico is.
 * What about a son or Daughter of Bacchus? Granted, one of the twins died in Olympians, but the other one has mostly been a Living Prop type. It's implied that the reason Bacchus/Dionysus doesn't really get around is that he loves Ariadne and keeps to her a lot.
 * Clovis is one of the seven. Remember at the campfire - "Some are here, some are not..." and Clovis says "I'm here!" We need a few more minor gods around - I hope there's also a Roman child of Discordia...
 * There's a better chance of Clovis being one of the seven, or at least, having a pretty significant role in the story, than you might think, because his father, Hypnos, is one of the only gods NOT descended from Gaia.
 * Riordan has confirmed that the seven demigods are children of Zeus, Poseidon, Hades, Hephaestus, Athena, Ares, and Aphrodite.
 * because no one will ever love Demeter. her kids never even get to give gifts/ useful hints to the questing campers. sucks for them, even though some of the myths have me convinced that Demeter could beat any of her siblings if she really wanted to. not that she would, but still.

There is an Oracle of Cumae.
Jason knows the last two lines of the prophecy, albeit in Latin, but it's obvious the Greek and Roman camps don't have association. Apollo works through both the Oracles, Delphi and Cumae, and it's possible the Roman camp got their prophecy through the Cumae oracle.
 * Jossed. There's not oracle at Camp Jupiter.
 * Not really, it just depends on your definition of the word Oracle.

Everything has a counterpart at Roman Camp.
Not just Chiron/Lupa - Jason (hero) plus love interest (Reyna) fought against the Titan Krios and some betrayer who was close to Reyna, plus help from teacher (Lupa). There's a counterpart to everyone at Camp Half-Blood.
 * Alternatively, Jason is the "Annabeth" of the Roman Camp, and Reyna is the "Luke". Piper will be his "Percy".
 * Jossed. Reyna is harsh but good. Octavian has a lot in common with Luke and Drew though.

Percy is not the Son of Neptune.
Poseidon hinted that Percy might have a sibling - Percy thinks he's joking, but Poseidon only gives a vague answer (but he doesn't directly say there is one). It's because Poseidon had a child through Roman aspect, who is the second book's Son of Neptune. Of course, that doesn't stop Percy from showing up.
 * And adding onto that, the Son of Neptune will be a villain, just to switch things up a bit.
 * According to the Other Wiki, Riordan has confirmed that the son of Neptune is Percy and not a Roman camper. Doesn't rule out Percy having siblings over there, though.
 * As of Son of Neptune, he's still an only child.
 * But Frank is a *descendant* of Poseidon/Neptune. And "son" can sometimes simply mean "descendant". So it could be Percy, or Frank.
 * or the horse.

Percy's Achilles weakness/fatal flaw will show.
It never got the chance to show in the first series - so this is Riordan's chance to fully flesh it out. Yes, Percy is still invulnerable, and it must help him. But while it's unlikely that Riordan will demonstrate the actual physical Achilles spot and kill our beloved Percy, remember that he still has an Achilles weakness - Annabeth.
 * That links in with the "Annabeth dies" theory above. It would go like this:
 * Annabeth dies.
 * Percy goes on a Roaring Rampage of Revenge
 * Annabeth is saved, the guy who killed her regrets it for the rest of his life.
 * Annabeth dies, they go to the doors of death to save her, and they fail. The legends only ever ended in a happily ever after for the original Perseus, and he was a son of Zeus not Poseidon. If Riordan wants the series to really mirror the original myths, Percy will have to fail, and fail miserably. His fatal weakness will thus not only get Annabeth killed (a la Heracles and Megara) but also himself. Only Jason will survive out of the twelve.
 * The series can't really mirror the original myths, because the standards of a story in ancient times were different than those of today. In stories today, the hero is allowed to die or worse, but aside from a few genres they can't fail.
 * Half jossed - current previews have him give up his Achilles heel, though he probably keeps his fatal flaw.
 * It's strongly implied that unless Frank acts as a counterweight Percy's fatal flaw will cause a lot of trouble...
 * Idea: In order to stop Gaea, one of the Seven must die, but Percy will refuse to sacrifice a friend; someone else (perhaps Frank) will need to force his hand.

Piper is an alternate-version Orihime.
Charmspeaking is part of being a Reality Warper, and she certainly saved Jason through her will.
 * I got really excited about this theory...but then I remembered that Piper doesn't have a brother. So much for Piper meeting her (dead) older brother as a Hollow shade of Hades.
 * Emphasis was on "alternate-version". Or maybe she does secretly actually have a brother. Hey, if Thalia gets to have one...

Reyna is a Faun!
I just like the Jason/Piper ship too much to let it die due to a new character.
 * Jossed.

Reyna is a daughter of Zeus!
Same as above, but who said the Pact applied to both Greek and Roman?
 * Er... that's incestuous, even for this series.
 * If Reyna was his sister, Jason remembering her wouldn't lead to him questioning how he felt about Piper.
 * Jossed. Reyna is a daughter of Bellona.

Reyna is the Oracle, and an ex-girlfriend of Jason's
Also same as above.
 * Jossed; there is no oracle. Octavian just slices open plushies to do auguries.

Grover is running for President of the United States of America
Taken from a throwaway comment from the previous series. Juniper probably will have made him.
 * Fridge Logic : Obama.
 * Who said this doesn't take place after 2012?
 * In The Movie, Grover is black. OSHI-
 * In the book he is not...

Percy is the titled 'son of Neptune'
At the end of the first book it is implied that Percy and Jason were switched. Percy and everyone at Roman camp will think he is Neptune's son and leader of the camp, having not coming to his senses yet. Hera actually replacing memories and a strong mist on everyone this time.
 * According to the Other Wiki, Riordan has said that the title mostly refers to Percy, but maybe not Percy exclusively, like how "The Lost Hero" could be referring to Percy or Jason.

Percy physically cannot forget Annabeth
If Percy forgets Annabeth, he loses sight of what makes him human, leading to Spontaneous Combustion due to being an Badass Abnormal.
 * Don't forget he still has the physical Achilles spot, though...
 * Alternate, the transfer to the Roman camp came with a washing off of the curse of Achilles, bringing him back down to normal mortal standards.
 * Confirmed, kind of. Annabeth is what he remembers, similar to Jason remembering Thalia.
 * Confirmed, kind of. Annabeth is what he remembers, similar to Jason remembering Thalia.

Jason is technically Zeus and Hera's son and the legitimate heir to the Olympian Throne.
Riddle me this: If Zeus were defeated by Gaia, who would reign over Olympus?

Deep down, Hera knows that Ares and Hephaestus are in no way fit to succeed their father should the worst happen. Jupiter's defeat might create a power struggle between Hades and Poseidon that would further split the pantheon in the face of Gaia's assault.

What's more, neither Athena, Artemis, Hermes, Dionysus or Apollo have any legitimate claim to the throne because the elder gods (Hera, Demeter, Hades, Poseidon, Hestia) would never recognize one of Zeus' bastards as rightful ruler. Hera wields more power over Olympus than any other god save her husband and she would naturally hold power over the chain of succession. Ares and Hesphestus would vote with their mama against their half-siblings and the Olympian base would be broken while Gaia scales Olympus like in God of War.

NVM didn't see below before editing (is Ares's Zeus and Hera's child.? I never heard of how he was born.) But I think Ares could defeat Zeus (I mean come on he's the god of war!!).

So how does Jason fit into this?

Because Hera was offered Jason's life and depending on how that was handled, it may be seen as kin to adoption. Jason is the rare son of Zeus spared Hera's wrath because he is "technically" her son as well. Hera is positioning her child as a legitimate heir and fail-safe against the chaos that would follow in the wake of Zeus' death. As a mortal hero, he can win the affection and loyalty of his aunts and uncles and position himself as a candidate for the throne. He and Percy stand to unite the Roman and Greek camps and thus have sway over a large number of demigods.Jupiter will be slain by Gaia and Jason will ascend, having won the favor of his family through his heroic deeds thus fulfilling the prophecy that a son of Zeus would rise above his father.
 * Thank you sir. This has to happen or I will proclaim the series as "crap".
 * Having thought about this for 24 hours, I have an add on to the WMG. All the gods will die, except Hera, Demeter, Persephone, Dionysus and Hestia. The Gods will be replaced by their most powerful children:
 * Poseidon = Percy
 * Athena = Annabeth
 * Aphrodite = Piper
 * Hades = Nico
 * Zeus = Jason
 * Ares = Clarisse
 * Hephaestus = Leo
 * Apollo = Unknown Apollo camper/Rachel
 * Artemis = Unknown son of Artemis (Filling the series's irony and chivalry quotas)
 * Hermes = Luke (People are coming back to life. Perfect time for Luke to redeem himself even more)
 * There's a gaping hole in this theory, though: Athena is explicitly defined as Zeus's heir by the laws of gods in the source mythology. Hera is Zeus's queen, but this does not actually make Athena a bastard child: Zeus's first wife was actually Metis, the mother of Athena. (This is why Zeus swallowed Metis in the form of a fly which caused Athena to come out of his head: he knew that if Metis gave birth to a boy, that boy would overthrow him as all sons overthrow their dads in king-of-the-god rules, the same way Zeus overthrew Kronos and Kronos overthrew Ouranos. Of course, surprise! Metis gave birth to a girl who turned out to be not at all interested in killing her dad.) In many of the poetry and materials regarding Greek mythology, Athena is Zeus's favorite child and heir.
 * You really think Hera is happy about that? You don't think that, whatever legitimate claim Athena may have to the throne, that Hera would rather one of her own children succeed? Hera would have very good reason to ensure that Jason succeed Zeus. Athena rising to the throne would mean that Hera is effectively deposed as queen of the Gods. As daughter or Metis, Hera would no longer have any place on Olympus other than goddess of marriage. What's more, Hera has made her business tormenting gods (Apollo, Artemis, Hercules) and has only gotten away with it because of her position as Zeus' wife. Now, lacking Zeus protection, might Hercules come for his revenge against his stepmother? Would Artemis and Apollo not seize this opportunity to get back at the woman who tormented their mother while she was suffering in labor pains? Would Athena be inclined to save Hera who now provides no use to the pantheon other than as a marriage goddess? Are there any lengths Hera would not go to in order to ensure that she maintains power over Olympus? Two ways this could be engineered is
 * 1) Athena is offed either in war with Gaia or by Hera who does away with her in Shakespearean fashion (See Macbeth or King Lear for examples of legitimate heirs being deposed). True, Jason may not be happy about this but with Athena and Zeus out of the way, he has to go along with it to prevent chaos and dissent amongst the gods. Hera buys protection by installing Jason on the throne.
 * 2) Jason turns out to be the more fit king. True, Athena is wisest of the gods but Jason could be stronger or more cunning(see Hercules for demigods becoming stronger than gods, especially sons of Zeus). To take a page from Norse Mythology, look at Odin. He was strong, but not stronger than his son Thor who could cleave mountains with Mjolnir. He was wise and a powerful sorcerer, but sources hold that Frigg, his wife, was better endowed (hurr hurr) in sorcery and divination. Odin was cunning but nowhere near as crafty as Loki, the Magnificent Bastard. What made Odin an effective king was his balance of strength, wisdom and cunning that the others lacked. Frigg might have been wiser, but she lacked the power to fight or the cunning to deceive. Thor would have been able to fight but would lack the wisdom to lead or the cunning to survive political situations. Loki would have been the master politician but a poor warrior and ruler. Now, going back to the Greeks, Jason might lack Athena's wisdom and good judgment, but he may yet gain adequate sense, good political savvy to make allies and power to defeat Gaia. If (when, let's be honest) Jason slays or imprisons Gaia, he could name his own tune and Athena might recognize him as a better ruler. Being the wisest goddess, she might logically decide that Jason is best suited for the throne and abdicate her claim to make way for a better ruler, one who possesses her beloved father's power, his adopted mother's cunning and wisdom gleaned from her tutelage. She might stay on as chief adviser to compensate for lack of his perfect wisdom. Athena (in Riordan's verse anyway) comes off as least petty of the gods and wants the best for Olympus.
 * Even Hera can't break the laws of Heaven. She's only queen of the gods NOW because Metis is kinda stuck in Zeus's literal head. There's absolutely no way Jason could legitimately take any ruling over the gods; if he did so illegitimately, he'd be VERY quickly offed. This is why the only legitimate threats to Zeus, both in Riordan-verse and source mythology, were people who had claims to the universe before Zeus (Gaea/her giant sons, Kronos, Ouranous, etc.). And if Athena deems him to be a better ruler...he'd be a literal God Mode Sue.
 * True, but Jason is already the long lost brother of an established character, possessed a Morph Weapon, is the son of the Big Three through a loophole in logistics, can control the weather, has two girls as potential love interests, suffered from Laser-Guided Amnesia that apparently made him forget that he was a badass praetor that wasted a Titan and belongs to Second Camp Half Blood...sound familiar? And this is still book one, so if Jason is going to get more powerful as the story progresses, there's not many ways he can go but up.
 * I'm sorry, but if the gods resent Percy for calling them out on and forcing them to change some of their bad parenting habits, after saving their asses from Kronos in a conflict that was caused DIRECTLY because of some of said bad parenting, there's no way the other gods will accept Jason, a mortal, as their new king, hero above all heroes demigod son of Jupiter or not.

The Union was the Greeks, and the Confederacy the Romans.
As I mentioned on the American Civil War page, Chiron mentions having trained Chamberlain in one of the earlier books; this would cause Unfortunate Implications, as the Roman-camp demigods are described as more warlike, untrusting, and violent...
 * In my mind this works out perfectly considering that the South and the Romans both had slavery.
 * Greeks also had slavery.
 * As did the Union.
 * I think the Greeks were the Union, but only because the Greek camp is in New York.
 * Or it's the other way round, mirroring the pattern of before where the Romans conquer the Greeks.
 * The civil war was before the fall of the British Empire: Olympus was in London at the time.
 * I agree with this theory.

Daedalus, Luke, Beckendorf, Silena, and Michael will return.
It's the Doors of Death! Anything can happen!
 * I'd put money on Luke making an appearance, at least. And Beckendorf and Silena would be awesome (Dead) Big Brother / Sister Mentors to Leo and Piper, especially since Jason already got his big sister back.
 * Tying this in with the whole Percy and/or Annabeth dies WMG above, I'm thinking one of them will die but makes a deal with Hades to seek out the tunnel Gaia bored into Tartarus to smuggle people out of the Underworld. He'll let them return to the living if they can destroy it behind them on their way back up. And since that's way too big a job for just one hero alone, que Luke/Beckendorf/Silena/Michael/Bianca coming with (maybe even Ethan, too?).
 * I don't think Luke would appear. In TLO, he said he thought he would try for the Isles of Blest, so he would be reborn and probably would have been reborn by now. However, I'm all for some of the main characters getting stuck and having Silena, Beckendorf, Michael, and Bianca pull a Big Damn Heroes moment as an opening. Well, that's how I picture it would happen.
 * Riordan has said that most if not all of the main characters from P Jat O will have a part to play in Heroes, so don't count on Luke being out the picture just yet.
 * Most, not all. Bianca seems to have already moved on and been reborn.

The Great Prophecy relates not to the literal destruction of the planet, but to Gaia's fall
Another term for "Earth" is "the world." She's the primordial goddess/incarnation of the whole world. Thus, she will fall to either "fire or storm" - Leo is Hephaestus' son and wields fire and Jason, as Jupiter's, wields a storm. Fridge Brilliance at its finest.
 * Not a bad idea, but Poseidon/Neptune is the god of storms, too. Remember that hurricane Percy summoned in The Last Olympian? So it could be him, too.
 * Also, what would that mean for the world? If the spirit of the West is embodied in the Olympian Gods, and their deaths would mean the unraveling of the entirety of Western civilization, then the fall of Gaia cannot mean her death because otherwise the planet is destroyed anyway. The most they'll be able to do is put her back to sleep.
 * The books have said many times that the gods and other immortals can't be killed, only pushed down or pushed back. "Fall" just probably means "fall back to sleep".
 * The "fire" part could refer to Frank. He is supposed to die watching his life stick burn in his hand.

Thalia will leave the Hunters at some point during the series.
Her reasons for joining the Hunters were pretty much exclusively to escape the prophecy. Now that Jason is back on the scene, she may find she wants to spend more time with her brother, something being a Hunter of Artemis would hardly facilitate. Couple this with the WMG above where demigods from the past books (coughLukecough) and Thalia may relinquish her immortality in favor of a normal life.
 * There was also the one-sided ship-tease between Thalia and Leo. Obviously, pretty much nothing came of that apart from Leo's musings, but I just got that feeling. All of Thalia's aging stops have pretty much put them at the same age, and it's as good a theory as any, for now. Plus, best friend's sister has been a time-honored romantic archetype in fiction for just about ever.

Dionysus may be off the bottle...
...but Bacchus sure as hell isn't. Jason and his friends are going to encounter Bacchus in his wildest state, gone mad by the fact that his Greek persona is forced into sobriety.
 * This is bloody brilliant.

My take on the 7 Halfbloods.
Twelve is a good number.
 * Jason
 * Piper
 * Percy
 * Leo
 * Artemis Fowl (Not a Half-Blood)
 * Random Dent (Not a Half-Blood)
 * Valkyrie Cain/Stephanie Edgely (Not a Half-Blood)
 * Carter Kane (Not a Half-Blood)
 * Sadie Kane (Not a Half-Blood)
 * A Son of Artemis
 * Annabeth
 * Reyna
 * Rachel (Not a Half-Blood)
 * Riordan has confirmed that the seven demigods are children of Zeus, Poseidon, Hades, Hephaestus, Athena, Ares, and Aphrodite.

One demigod will be a child of Eris.
Because she should be in this series, darn it. But for a little twist, the Eris-child will be a good guy (because hey, you can always cause discord amongst your enemies instead).
 * Jossed

Piper's ability to speak French because it is the language of love is actually a brilliant Stealth Pun.
What's another word for love? Romance. Much like the Greek Demigods' ability to understand Greek, Roman demigods have the ability to understand Latin and languages that grew out of it. Jason doesn't understand French only because he has the natural aptitude for any of the other Romance languages (Italian, Spanish, Portuguese, Romanian, etc...). The Greek demigods get the short end of language mastery because there weren't any widely used evolutions of Greek.
 * Heck I bet you anything that children of Venus are able to speak all romance languages, too.

Jason and Thalia have another sibling...
...because there have to be three Graces.
 * That might not be such an Incredibly Lame Pun as much as Fridge Brilliance. We've already been told enough times that three is a strong number to the Greeks, so why not? The name isn't exactly subtle, and he's hardly shied away from such obviousness before Rachel Elizabeth Dare indeed.

Percy will be given the ability to speak/understand Latin out of necessity.
Think about it. If the demigods over at the Roman Camp Halfblood get their prophecies in Latin, they probably use it often enough that not being fluent in it would be a major red flag that Percy doesn't belong. Definitely a major stumbling block on the whole getting-them-to-trust-him thing if he can't really understand them half the time.
 * Although, Percy having to put so much effort into learning Latin might go a long way towards showing the Roman demigods that he's genuinely willing to adapt to their culture and their acceptance of him.
 * Percy at least knows the bare basics of Latin. Recall, Chiron worked at Yancy Academy as Mr. Brunner, the Latin teacher. While Percy sucked at Latin, he might know just enough to be able to pick it up in a life or death situation. Which also begs the question of why Chiron would teach Latin when he was looking for Grecian demigods, as opposed to some sort of mythology class or history class (which are offered in some form or another in most schools), unlesss he knew that sometime in the near future, the Roman and Grecian Camps would have to meet. He obviously thought there was something special about Percy, because he doesn't seem to just drop everything and take up a teaching position every time there's a new demigod in town. Maybe Chiron has known all along that this would happen.
 * Latin looks better on a college transcript than Greek. Heck it wouldn't surprise me if Lupa and Chiron knew Latin and Greek to talk to each other.
 * Although one would have to wonder how much of his Latin lessons he remembered after almost a decade without them (as far as I know, he didn't take them after book one). Keep in mind that though Latin was the official language of Rome, Greek was still the lingua franca.
 * Or he's been going through the exact same thing that Jason went through at Camp Half-Blood, his memories coming back in bits and pieces, his uncanny ability to speak Ancient Greek, the fact that he might still be under the Curse of Achilles? It wouldn't be hard to spot that he had a lot of things different about him.
 * Just an addendum: In the first book, when Percy is fighting Mrs. Dodds, he shouts (inexplicably in Latin, prolly powered by the same thing that makes him speak Ancient Greek) "Eat my pants!". I assumed it was Continuity Drift given that Riordan prolly didn't conceive this series by this time. But you never know...

Ouranus will appear.
Aphrodite mentioned that she was created from his remains, but as we know from Kronos, even if gods are cut up into little pieces it doesn't mean that they're really gone. True, he's supposed to be even worse than Kronos, and pretty much everybody hates him, but Ouranus would be a pretty powerful ally against Gaia. Or he could attempt to use the coming war to reinstate himself as lord of the universe in a truly epic melee a trois!
 * Or maybe we'll get to see those events from his side - history's written by the winners; Gaea may have been the one to engage in Disproportionate Retribution towards him and not him towards their children.

Hypnos will be a villain and wake Gaia, or help the heroes in putting her to sleep.
The presence of the Hynpos cabin with all this talk of keeping Gaia asleep is too coincidental to be left alone. Alternatively, Hypnos will want Gaia to sleep, but Somnus will want her to stay awake and keep the gods and demigods in line.
 * Why would the god of sleep want to wake the worst enemy of the gods? He wouldn't really get benefits from a chaotic world ruled by Gaea.

Tantalus will come back from the dead.
And he'll try to eat the demigods.
 * Is he technically dead anyway? With the gates of death opened he does seem like an obvious choice to bring back, given his mythical history.

Aphrodite's ploy to get Jason and Piper together will backfire.
If Jason does have a relationship with Reyna (and he probably does, added romantic drama all but demands it), then the Roman demigods might see his turning away from Reyna in favor of Piper as Jason's being manipulated and corrupted by the Greeks, something they're not likely to take kindly to. Face it, if something similar happened to Percy to the same degree that it's happening to Jason, the Greeks would be up in arms at Annabeth's defense in a heartbeat. The fandom certainly has.
 * He didn't, he and Reyna were rather close and probably headed in that direction but Hera/Juno got in the way before something could actually begin

Percy and Jason will duke it out to be leader of the two camps on the quest to Greece.
Percy's not going to just had the role of leadership over, so it'll have to come down to a fight at some point or another. My guess, Percy will win, because, you know, he's Percy, but will give it to Jason in the end anyway. There's certain to be times during the quest when he'll have to go off alone and be the badass One-Man Army that he is, which is when he'll need Jason to be able to lead the others. So why still the fight? Because Percy still wanted to make sure that Jason was up to the job.
 * I doubt it. I'm betting that Percy will lead the Greeks and Jason will lead the Romans.
 * I'm actually going the other way. Hera isn't going to return their memories until the mission is over so they will know their friends at the other camp better then they do their original friends. This is shown by the end of the book when Hera tells Jason "You will be a leader at this camp".
 * Jossed; by the end of the second book Percy has all his memories back and it's been long enough that it's safe to guess Jason has regained all of his memories as well.

A lot of this is Alternate Character Interpretation

 * Gaea was portrayed as a more positive light because well Oranous locked away their children as Disproportionate Retribution for being "ugly". Instead; it might be that the children who were locked away in Tartarus were grounded for doing something wrong but Gaea is the one who engaged in Disproportionate Retribution and talked Kronos into becoming Lord of the Universe.
 * Heck for all we know; history's written by the winners - we saw it from Gaea's perspective because she spoke Kronos into killing (and slicing up) Ouranos - Ouranos, if he were still alive, would probably tell a different story.
 * Roman aspects in themselves are all Alternate Character Interpretation because they were assumed to be more disciplined.
 * When Hera acted nicely towards Percy and Annabeth, she meant it and only sent the cows after Annabeth because she acted mean to her. Hera probably realized sometime that she wouldn't really be setting a good example if she threw her children off of Olympus because they looked ugly or taking her frustration towards Zeus off onto his own children, but having a nasty reputation for that probably doesn't make it easy - I mean it's kind of hard to change your personality and attitude if people keep bringing up the past. Heck, Hera probably mentioned that she didn't have any heroes of her own and Zeus probably said, "Hey, I got one I conceived when I was Jupiter, why don't you try your hand at it?". Or he did so after Hera made him sleep on the couch for a couple decades. :P
 * Apollo is a Spoony Bard. His greek aspect is like Demyx or Johnny. His roman aspect is more like Ricardo.
 * Apollo doesn't have a different Roman counterpart. He is Apollo with the Greeks and in mythology, Apollo with the Romans.
 * Fair enough observation, although there is the little matter of Gaia  that kinda ruins this theory as applied to her.

Will Keith Kellogg and/or C.W. Post were children of Demeter's Roman Aspect
The word "Cereal" comes from "Ceres" anyways.

Jason's fatal flaw is envy.
There are a surprising number of references to it in the book:
 * Envious of Annabeth's devotion to Percy.
 * Envious of how hard Thalia is looking for Percy, who she's not even related to.
 * Outright jealous of a smile Piper gave to Lit, Midas' son.
 * Jossed, he has the same fatal flaw as Percy, personal loyalty
 * It isn't confirmed yet.

Son of Neptune will not feature Jason from that start
They mentioned quests running parallel? Well, Son of Neptune will be Percy's quest, starting from his arrival at the Roman camp, parallel to Jason's in Lost Hero. If Jason, Piper and Leo make an appearance, it won't be until near the end of the book.
 * Given what Hera said about Jason finding his own way back and needed the time between to deepen his bonds with Piper and Leo, I don't think the two camps/quests are going to meet up that quickly. There might be some kind of cameo, i.e. Festus or the helicopter flying over, but it won't be a face-to-face thing. If Percy goes on a quest, he'll have Roman demigods with him who would recognize Jason immediately.
 * The first chapter has been released on the website and has shown.
 * It doesn't have to begin

Percy and Jason have already met.
We know that Percy was missing for three days before Jason showed up, right? And given the potential danger in Hera's little Batman Gambit, it's likely both boys were taken at the same time. So what were they doing for three days? I have no idea, but I'll bet you anything they were in the same place together at the very least.
 * Except Percy and Jason were taken months apart. It is mentioned that Jason was taken at some point in October, while Percy was taken at some point in late December. While there is a little bit of overlap there, Jason appeared on that day because that was when Annabeth and Butch were coming to pick up Leo and Piper. Also, after getting his memory back, Percy still doesn't know who Jason was besides seeing someone he thinks is Jason in his dream of Camp Half-Blood working on the Argo II.

The series will end in tragedy and heartbreak.
Riordan has Percy mentioning how often the Greek heroes' stories ended in tragedy, all bar Perseus. This is not coincidental, and as the latest series continues this will become even more obvious. Percy's fatal flaw will begin to seriously cause serious damage, including Annabeth's death. Jason will be jealous (see WMG above relating to his fatal flaw) of Percy and this rivalry will damage any quests. Clarisse's little prophecy about 'bear arms to the Doors of Death' meaning the demigod camps will fight will be true, resulting in her death fighting a child of Mars (we've already seen how much she fears Ares, now imagine if an even more warlike child of Mars was to face her, she wouldn't stand a chance.)

Eventually Percy will die, the camps will be destroyed and the demigods scattered around the globe or killed horribly, Chiron will have lost his purpose as a teacher and fade away, Piper will die as a result of Aphrodite's meddling with Reyna and Jason, Leo will die so we have the symmetry of a son of Hephaestus dying heroically at the beginning of the final book, Grover will fade into nature, and the best that Jason will be able to do is send Gaia back to sleep before he curses at the heavens that his parents were so cruel, all while the gods utterly ignore everything that's going on, or possible even begin their own war in heaven meaning that Western civilization will be sundered for years.

Because otherwise, it just wouldn't be true to the spirit of the original myths.
 * True, but considering Percy is basically the literary version of Riordan's son, this is extremely unlikely.
 * Also keep in mind that the Greek Gods have become Americanized. American demigods include people like George Washington and Harriet Tubman. Greeks liked tragedy and despair. Americans prefer a nice happy ending.

Jake Mason is the Grandson of a certain other Mason.
Alex Mason, son of Apollo, settles down and has children. One of his daughters, a weapon designer or something, catches the eye of Hephaestus, who doesn't care she's his half-grand niece.

Piper doesn't like Jason, but...
Leo. Think about it! When they find a way to clear up Piper's Misty Mind, her feelings wasn't for the Golden Lad, but for her Annoying friend. Then Love Problems begins. But it's like Silena/Beckendorf all over again!

The children of Apollo have the ability to switch back and forth between Greek and Roman.
Derived from the fact Apollo's Roman name...was Apollo.
 * Percy might be able to do something like that, too. Remember how Kronos called him "too changeable"?
 * That might have just been referring to Poseidon's changeable nature.
 * Or Percy is badass enough to be Greco-roman instead of either.

Jason was revived/possessed by a god.
After Piper brought Jason back after he "died", his eyes were solid gold for a second. Just like Luke's were when he was possessed by Kronos. That can't be a coincidence.
 * or worse Ouranos is still kicking it and possessed him to some degree. Or it could have been a side effect of being brought back inches from death's door(hehe) the way he was.
 * The fact that he was revived at all suggests that. Perhaps he, like Percy, was offered the choice of becoming a minor god and accepted.

Gaia's waking up because of the letter that Sadie gave to Geb, Egyptian god of the earth, from Nut, the goddess of the sky.
Riordan has stated that both PJO and the Kane Chronicles are set in the same universe, and The Red Pyramid happened just after The Last Olympian, right? The pantheons are supposed to be separate, true, but who's to say one god can't have an effect on another when they're both gods of the same thing?

Percy is the son of Neptune AND Poseidon.
A major hallmark of a Roman demigod is being able to speak Latin fluently right? In The Lightning Thief when Percy is fighting Mrs. Dodds on the bus, he shouts "Braccas meas vascimini!" or "Eat my pants!". Not exactly fluent, but! Sally Jackson is a clear-eyed mortal, she can see things as they really are and so might have been able to see through the whole Poseidon/Neptune as separate aspects thing. Percy might have have a bit of Roman blood in him already. It would explain why demigod children of clear-eyed mortals are more powerful that normal (Percy, Luke, Nico), because they have a bit of their godly parent's Roman aspects in them along with the Greek.
 * There is just one flaw to your idea. Latin is still taught in a number of schools. So Percy knowing Latin is more of a result of learning it (from Chiron no less) then as a result of "in the blood", so to speak.
 * Remember though, Greek demigods brains are wired for Greek, and they have problems with ALL other languages-this would include Latin as its not related to Greek in any meaningful way. It's been a while since I read Lightning Theif, but as I recall Percy was one of the better kids in the class with Latin, or at least did signifigently better then with English.
 * Also note that in said passage, Percy professed that he had no idea how the Latin came out of his mouth (in a similar vein to his Greek).

A child of Apollo will be the bridge between the two camps.
Apollo doesn't exactly have a Roman half. His child would be more fluid between the two camps.

Piper is a daughter of Venus.
It would certainly explain why she is so much more combat oriented and different than her siblings.
 * But unlike Jason, she feels like she belongs in Camp Half-Blood and knows the gods as Greek. Her difference is probably just a way to break the Aphrodite cabin stereotype. Her time in the Wilderness School would also explain some of her battle skills.

Suspicions on theories towards Child of Artemis/Diana
If I'd have to guess, the seven will be From CHB From RCHB This is mainly on the views of each pantheon, for the Greeks were artistic sailors, hence the sea, love and smithing, while the Romans were strict and waring, hence the king, archers/healers and warfare. The seventh, however, will have to be something in between. Hence where a child of Artemis or Diana would come in Did anyone notice how Artemis was a little....nicer with guys in the Titans Curse? As Nico, Grover and Percy are still alive and, sentiently not jackelopes. Though she still doesn't say, love them, she appears to only really harm those who stumble and act to what she views as boys (Eg, flirting, lustful, dumb....get the drift) The fact that this didn't happen, and that she actually went out of the way to save them, shows that she obviously has mellowed out at some point towards them. So, the idea of her having a child, easily a son somewhere is plausible Based on how this isn't common knowledge, it is possible he/she is some sort of loner, and thus not affiliated with Greece or Rome, thus a balance. Though how said child could have been born without oath breaking will be a curiosity
 * Percy (Poseidon/Neptune)
 * Piper (Aphrodite/Venus)
 * Leo (Hephaestus/ Vulcan)
 * Jason (Jupiter/Zeus)
 * Unknown (Apollo)
 * Unknown (Mars/Ares)
 * Luke will come back from the dead.
 * Riordan has confirmed that the seven demigods are children of Zeus, Poseidon, Hades, Hephaestus, Athena, Ares, and Aphrodite.
 * Hazel, daughter of Pluto is one of the seven

Percy gets in a huge fight with all the Romans when he first stumbles into camp.

 * Jossed, unless you count teaming up with Frank & Hazel to squish the cocky half of the camp during the war game.

The Roman camp is a military boarding school for demigods.
It's implied that, in general, a single Roman demigod is more powerful than a Greek demigod of the same parent, probably powerful enough to require being protected at camp year round.
 * Not exactly... it's an entire community with the legions as guards against monsters, complete with descendants of demigods.

Percy will get a new weapon made out of Imperial Gold.
Since Gaia opened that tunnel into Tartarus, plain old Celestial bronze isn't doing the trick anymore. And it would allow Percy to compete with the Roman campers on more equal terms weapon wise.
 * Jossed. Percy uses Riptide throughout the book.

Only Jason wears purple at the Roman camp...
...either because he is the son of Jupiter or because he's the praetor. Red was THE color of the Roman army, so most the Roman demigods probably wear that.
 * Jossed. Everyone wears purple. Interestingly, both orange and purple ARE half-red though.

The fishing pole that Poseidon left behind on the beach in TLO is a new trident for Percy
. That thing was strong enough to withstand a monster fish pulling on it, so why else would Poseidon leave it behind?

Nico, after a sudden appearance at the end of the third The Kane Chronicles book will drag the Kanes into the third Heroes of Olympus book
Nico radiates death, like a certain god that Sadie might just have a crush on. The unresolved tension between her and Anubis will boil over convincing her that Nico's a host of Anubis and she'll drag Carter along to help prove it. She'll keep insisting that everything is still Egyptian even as Carter yells at her it's not until there they are at the River Styx talking to Hades.
 * Okay, this now needs to happen. And with The Serpent's Shadow coming out months before The Mark of Athena...

Clovis will have an instrumental role in bringing down Gaea.
Somebody needs to put her back to sleep...

Percy has/will have a quarter-sister.
I'm noticing that in The Lost Hero Thalia, daughter of Zeus got a brother who's the son of Jupiter. And now in The Son of Neptune Nico helps revive his quarter-sister, Hazel, a daugter of Pluto. To complete the symmetry Percy will get a sister who's a daughter of Neptune opposed to Poseidon in the third book.
 * The term quarter-sibling denotes that one child was born to the Greek form of a god while the other was born to the Roman form of the same god.
 * Technically,

Alaska is home to the Norse gods.
Despite being part of America, Alaska is apparently beyond the control of the Greco-Roman gods. Why? Simple- it's the domain of the Norse gods. The gods may or may not know this. Evidence- Percy feel the ocean is more primal there, there are frost giants, and it's far north. A sort of Mist thinly divides the two mythologies, akin to how there's a barrier between the Greco-Roman and Egyptian mythologies but at times the division can be felt.
 * This will be expanded upon once Rick Riordan finishes the Kane Chronicles. And one of the other NYC boroughs will be the seat of the Norse gods though to continue the pattern.
 * Uhh...Alaska is home to the Inuit natives, too, you know. I don't think the Norse ever got that far. Greenland and Iceland, yes. And even parts of New England before it was New England.
 * The Greeks never made it to America and neither did the Egyptians, but they both have Gods in the country, and they were both given reasons (even though the main reason is that Rick is American so is going to set the books in his own country).

Ella is autistic
The way that she is described fits a lot of the general indicators of autism,.
 * Actually, her ability to is an indicator of an eidetic memory, but otherwise, that could be the case.
 * Hmm. If ADHD and/or dyslexia means you're part god, does autism mean you're part harpy?
 * Not all demigods have ADHD and/or dyslexia, Frank only seems to be lactose intolerant. Maybe Ella is a special case, even for harpies. It could be why she was mistreated by the other harpies.

There's going to be a lot of tension between Leo and Frank.
For more reasons than one:
 * Leo is
 * . That couldn't possibly go wrong.
 * Although it's not strictly essential to the theory, it's worth noticing: Leo is the son of Hephaestus/Vulcan. Frank is the son of
 * Made of Win
 * So history repeats itself through the children. Too bad Hazel isn't a daughter of Venus as now we can't say generation xerox.
 * Really too bad. I can imagine Frank and Leo being total dorks together and stuff.

The reason Jason didn't die is because Thanatos was already chained up
By the time Jason, Piper and Leo set out for their quest, monsters are already instantly reforming. Jason came back because he could slip past the border control. And Piper's charmspeaking helped the process.
 * YES remember when gwen said "walked through back door".... Piper's voice made Jason turn around and see the open door.

Bacchus isn't part of the roman twelve.
Instead it will be Vesta. The roman scheme involves six gods and six goddesses if Bacchus was there he'd unbalance things.

Gaia won't be the true final villain. Ouranos will be
What better a twist than for the supposedly evil Ouranos to be brought back and get revenge on Gaia for her betrayal then continuing the war on Olympus so that he could take his proper place as King once more. On top of that he embodies the sky so he should have power over the weather such as bringing about storms. That's right the second line of the prophecy is about the return of Ouranos. The foes bear arms to the doors of death could be Ouranos fighting against Gaea. Other hints include:
 * Atlas mentioning the Sky still longs to embrace the Earth in Titan's Curse
 * After being told the story of Gaea in the first Heroes of Olympus book Piper looks up at the sky as though it had a face.
 * When Jason was brought back his eyes were golden for a split second, while that could be simply from being revived the way he was it could also be that he was possessed by Ouranos(who could still be weak from being sliced up and thus hasn't taken control)
 * The Second Great Prophecy. Just second great prophecy.

The third book in series will be from Annabeth's point of view ONLY
Because we already have established the other 6 demigods in the prophesy, Annabeth is the only one left to have her own point of view. What the inside of her head actually looks like is a different story.

What Annabeth's inner head is really like...
We've never actually gotten a chapter in her point of view, and considering that people can be very different on the inside than what they show us on the outside, she could be like: Or entirely different from how her friends see her.
 * Snarky Ellen Page as Juno
 * Panicky bookish nerd girl like Hermione Granger
 * Adventurous and plucky like Coraline

Zeus being stubborn and closing Olympus is a deliberate gamble to trick Hera into bringing the two camps together.
Like Hera, Zeus probably doesn't like that his family is spit between two places. But as Jupiter, he has to abide by his own rules. So he's pretending to be making a really stupid decision in order to trick the other gods into doing it for him.
 * I think your giving Zeus far too much credit. Hera stated Zeus has a long history of putting his pride before everything. Throughout the books Zeus has repeatedly made the wrong decision on major issues. There is no "rule" that states Jupiter has to keep them apart or to indicate things have to remain that way. There was no rule that stated Jupiter had to seal Olympus. It was just Jupiter's sheer stupidity and pride.

The reborn version of Bianca di Angelo will make an appearance at some point
Nico mentions that she's no longer in Elysium because she chose to be reborn. Although, logically, her new form can't be older than about one year old, I'm betting that either Nico will find her (and be angsty because it's not really her) or we'll get some hint/implication about who/where she is.

Hazel is the reincarnated Bianca.
After the Doors of Death are closed, Hazel, who should be dead, would logically have to die along with all the other escaped Underworld denizens. But it doesn't look as if the author is going to go that route, so we can only conclude that Hazel has gained the right to live again. Somehow, Bianca managed to chose her as a way out of the underworld.

Nico probably doesn't know about this--but maybe he does. Or maybe he suspects the truth. Which would explain his awkwardness around her, almost calling her "Bianca", etc.

Just to add interest to this theory, Hazel is black, and the name "Hazel" implies a dark color, while the name "Bianca" means "white".
 * Or, you know, he could be almost calling her Bianca because he still misses his big sister. Keep in mind, Nico is still just a thirteen kid. In some part of his mind, he thinks that bringing Hazel back will be like having Bianca back, but the reasonable part of his mind knows that she's not Bianca. Not that that keeps him from almost saying it. I think that a lot of people forget how young Nico is, and how confusing it must be for him to go into the Underwold, looking for his big sister, who he's now older than, realizing that she's chosen to be reborn, and finding another child of Hades/Pluto around Bianca's age.
 * That's the way it appears on the surface, but something else could be going on that we're not aware of. Remember, Nico still hasn't told Percy and Hazel everything. He has a lot of explaining to do. - OP
 * Jossed, Hazel, Bianca and Nico all lived at the same time in World War 2, she can't be Bianca's reincarnation

Hazel will draw, or has drawn, prophetic pictures, just as Leo did when he drew the Argo II as a child.
I doubt that Pluto would bring her a box of crayons and a pad of paper for no particular reason. Plus, having Hazel make "crayon drawings of destiny" would contribute to the speculated connection between her and Leo.

The Ultimate Sacrifice Percy has to make will be to give up a certain set of memories.
It has been stated that Percy will have to make a great sacrifice where his fatal flaw (personal loyalty) will get in the way. It is also stated that the Greek and Roman camps will have a gap bridged between them to bring them together in unity.

It is NOT stated for how long this unity will last, or that someone has to die in order to fulfill Percy's part of the prophesy.

My theory is that after the last gigantic battle uniting both of the camps, the Gods will state that they themselves cannot be 2 people at once for physical reasons (it's just impossible to do without erupting into pure energy) and emotional ones (suddenly their children are faced with the fact that their parents aren't all of what they said they were, or how they remember them.) In order for everything to return to the status quo, Percy must make the sacrifice to forget Camp Jupiter. Since he is now the main link between both worlds, making him forget Jupiter would cause everyone else to forget what happened to them too. Percy will have a very hard time with this, because odds are that campers from rivaling camps will have formed relationships with one another (both friendship wise and romantically), and the fact that Jupiter has an entire world built on keep Demigods and their families safe for life, something that his camp could desperately use. Seeing how this could hurt his friends, and how Percy himself has already built very strong bonds with campers at Jupiter, this will be an ultimate sacrifice for him.
 * Percy is the main link? What about Jason, he is just as powerful as Percy is and has spent time in both camps as well. The ultimate sacrifice is going to properly be
 * To counter that, though I think you make a good point, think of it in these terms. 1) This entire series, as well as it's predecessors, is aimed primarily at kids slightly younger than the original Harry Potter demographic. 2) The books are published by Disney Press. 3) Percy being the head hauncho and big damn hero is almost a tradition. Him being a bigger link than Jason wouldn't be unusual. Percy has already made it pretty clear at Jupiter that he's a big deal, with already getting a high rank and permanent status, while Jason returned to praise and congratulations.  a main character with some pretty epic powers and a romantic link, seems very unplausible considering who's printing and the demographic. If they really wanted to go THAT route, they'll probably have it be someone who wasn't such a big deal in this series, like
 * Considering how important Grover was in the last series, I highly doubt that Rick would kill him off at this point. He'd be a lot more likely to kill Frank, considering how much trouble the story takes to set Frank up as a person destined to live a short but heroic life.
 * There was plenty of death in the previous series. To quote above, there was a character,, that had pretty epic powers,   and was the only character in the original series to have a romantic link. The only difference is that   wasn't a main character, but he still appeared in pretty much every series in the original series and he was liked by everyone at Camp. Plus there were plently of other deaths during The Last Olympian. And on a side note, Grover can't be killed remember because the empathy link would kill Percy as well (unless that is the plan of Rick)
 * To put it into perspective, there is difference in killing off Fred Weasley and killing of Ron. Would Rick really go that route?

Frank will die a heroic death...
...and Hazel will be forced to choose between joining him in the Underworld and staying alive with Leo.
 * For added angst, Frank will encourage her to stay alive.
 * Somehow, Franks  will not be able to catch fire by itself and Leo will be required to light it for him.
 * Alternatively, they both die, mirroring Silena and Beckendorf in The Last Olympian. Hazel will probably die first, maybe in a Heroic Sacrifice to delay Gaea's plans;, either by Gaea or Porphyrion's hand in the last or second-to-last book. Probably Gaea, by turning Hazel's powers against her like she did with Leo. For added cruelty, and as another callback, this will happen just after they establish themselves as a couple. When Frank is about to destroy Hazel's killer using the power of  , said killer will beg for mercy... at which point, Frank reminds them of Hazel's death,   Hazel and Frank are reunited in death, and end up in Elysium together.

The Mark of Athena Prophecy is

 * Annabeth is the sole narrator/focus character of the book and something happens that make hers in/famous in Rome/New Rome
 * Annabeth's part of the book involves her travelling alone from the others and Athena will be more deeply involved than she ever had before.
 * or some combination of the previous two

All of Aprhodite's children (or at least some of them) actually have the potential to be Manipulative Bastards
Aphrodite's Heart Is An Awesome Power spiel in Lost Hero got me thinking: what if her children were able to manipulate people through the power of love to make up for most of them not really participating in battle games and such? We already saw how Drew used her potential- what if other Aphrodite children were able to do things such as pretending to like someone just to get something, or could read emotions as easily as...I dunno, Izaya? Then they would REALLY be a force to reckon with...

The Mark of Athena will be very different from any of the other books.
The Mark of Athena will be very broken up in stream of conscience style, and in first person. It will be Annabeth recounting what was going on during Percy’s absence from the start of “The Lost Hero” through “Son of Neptune” and then afterward when they find Percy. It will fill in the cracks between what was going on at Half-Blood with the Other 3 heroes, as well as what she was doing when they were on their quest.

This is not Percy's first life.
This is actually Percy's third life as a hero trying for the Isles of the Blest. He'll end up trading his spot there for something/someone precious, similar to how Hazel traded her spot in Elysium for her mother not going to the Fields of Punishment.

After the final battle in the last book of the series, the whole world will restart from Annabeth and Percy's last night before the whole thing happened, but with some key differences
My theory goes like this: After the final big battle, a lot will have gone wrong. Many people will be dead, including someone, or more than one person who was probably a main character. Also, Percy and Jason lost 7 months of their lives. That's a lot, considering that Percy at least has a life outside of camp, and Jason being treated as a head of his camp. But, the Big Bad is dead, so that's a plus. However, the Gods from both their Roman and Greek stand points think that the demigods deserve a reward for doing the God's bidding. So, the decide to restart time from right before everything went crazy. Only this time, all the enemies and places they dealt with are still dealt with, so they don't have to redo everything. Instead of Percy and Jason being taken away, they each receive a letter from the opposite camp's directors telling them to meet, thus creating a partnership between both camps.
 * That would be kinda cheap. If Rick did that, I can imagine a lot of fans (including this troper) being really annoyed that effectively none of the series happened. Plus, the Roman Camp has already proven that they really only accept someone if they have proved themselves in battle.

Pollux, son of Dionysus, will help Dakota overcome his drinking problem.
Dakota probably drinks as a way to invoke his father's power (to sustain his godly half). Pollux will teach Dakota how to use his divine gifts as a son of Bacchus, so Dakota won't have to rely on drinking kool-aid anymore.

Octavian isn't evil.
Just a thought. He does show some signs of sociopathy, though, which is frightening.

Or perhaps not truly evil, but definitely not purely good either. I mean, she all but gives Percy a We Can Rule Together speech when she's talking about how powerful they could be together if he became praetor. No one who's a true "good guy" in a fantasy series talks that way. That scene had definite tones of temptation, combined with a little Ambition Is Evil.
 * Bellona, the Roman Goddess of War, is her mother. It must be made of note that Bellona was one of the original Roman Gods from before Greek influences took over. In other words, when the Gigantes destroy the places of power of the Olympian Gods, Bellona will be unaffected...

Annabeth hits Percy
Bets are on: how much would you wager and where would you wager it-- that Annabeth punches Percy upon seeing him again, or kisses him?
 * Both.
 * Ten drachma says she punches first, then kisses. We've already seen something close before after the Mt. St. Helen's incident.

Sally and Paul didn't pick up the phone in Son of Neptune because they've been kidnapped by Gaea.
Gaea's already messed with the mortal parent of half of six known demigods of the Great Prophecy, and directly caused the deaths of two. There's no way she's going to just leave Sally and Paul alone when Percy's the biggest player in the war.

It will be revealed that Jason has a fear of water, or at least the ocean, and it will cause some trouble during the quest.
I mean, Percy already has a fear of flying. It's entirely possible that Jason isn't too fond of the ocean, due to it being Poseidon/Neptune's domain.
 * The Son of Neptune demonstrated that all Romans are not that fond of ocean travel to begin with and try to avoid it. But considering that Jason has spent nearly six months with the Camp Half Blood crowd and he knows that he has to travel over the ocean for this final mission, he may have over come that.

Sisyphus will appear in the books and be reveal to have been the one to chain Thanatos
He did it before and then Ares intervened except this time it was Frank.

The Mark of Athena will be a...
Curse in the form of a partridge, because of the whole Daedalus-thing.