Avatar: The Last Airbender/WMG/Plot

Ozai is responsible for Lu Ten's death.
Ozai seemed very prepared for the occasion. Also, if he would readily kill his own son, killing his own nephew shouldn't be that hard. Also, Ozai presumably green-lighted the plan to kill off an entire battalion of new solders, so he probably doesn’t care who is killed in battle. Maybe he had Zhao kill Lu Ten. Zhao, being as close to Ozai as he was, must have known of Lu Ten and Zuko's close friendship. He could have done it only to spite Zuko.
 * Kinda hard for Ozai to have that kind of control over a battle happening on the other side of the world.
 * It would not be too difficult for Ozai to arrange a covert assassin to kill Lu Ten in the middle of some heated battle. Everyone would then assume he was killed by enemy forces.
 * He's prince of the Fire Nation which is winning the war. Also, Zhao's been known to go to such lengths for the sake of making some kid's life hell.
 * If Ozai did arrange Lu Ten to die, it was probably for a better reason than to "spite Zuko": he did it to ensure that the coup where he claimed the title of the Fire Lord would succeed. Ozai knew how important Lu Ten was to Iroh, so he realized that by arranging Lu Ten's death he could emotionally crush Iroh. Ozai presumed that Iroh would become so depressed he'd start acting in ways not fitting to a future Fire Lord, and the depression would also ensure he wouldn't oppose Ozai's coup. Also, as pointed out in the series, Lu Ten's death meant Iroh would have no legitimate heir once he became Fire Lord. As we see, Ozai's plan works almost as he thought it would: Iroh becomes depressed by his son's death and abandons the siege of Ba Sing Se. Most likely Ozai couldn't have foreseen this specific development, but it certainly fits his plan, as it undermines Iroh's capability as a military leader, a capability that is extremely important to a Fire Lord. What Ozai didn't foresee, however, was Azulon's unshakable loyalty to Iroh. Even after Ozai presented his case why Iroh should be passed over for the throne, Azulon refused to do so, which forced Ozai to arrange for his death. Ozai's original plan worked in one aspect, though: Iroh never challenged his claim for the throne. At this point Iroh was probably a better fighter than Ozai, so if he had opposed Ozai's claim things probably would have lead to an Agni Kai, which Ozai knew he would have lost. So by arranging Lu Ten's death Ozai ensured Iroh would not stand in the way of him becoming the new Fire Lord.
 * Of course one could ask why Ozai didn't arrange for Iroh to be killed in the siege of Ba Sing Se instead of Lu Ten, but there could be many reasons for that. First of all, Iroh was undoubtedly a better fighter than Lu Ten, and as a general he would have stayed behind the battle lines, so arranging his death in a way that wouldn't look extremely suspicious would have been much more difficult. And it's perfectly possible there was still some remnant of love and loyalty for his brother left in Ozai's twisted mind, so he simply couldn't bring himself to have Iroh killed, even if he could do that to his son.
 * If Iroh was killed, then Lu Ten would be the next Fire Lord. Even if Iroh didn't become super depressed and give up the throne, he is now heir-less, making his 20-years-younger or so brother next in line after him. This is sort of the angle Ozai was trying to pitch to Azulon, but Azulon didn't react how he expected. Anyway, maybe Ozai was planning on killing Iroh next, though probably at a time when it would be less suspicious. Instead, Iroh gave up the throne so Ozai didn't need too.

Getting struck by lightning in Ba Sing Se was a giant cosmic Get a Hold of Yourself, Man!-moment for Aang.
Some powerful spirit, the goddess of love or something, really didn't like how he was trying to give up his love, and decided to use Azula as a medium to snap him out of it.
 * This spirit obviously has no concept of overkill, seeing as Katara had to bring Aang back from the dead after that.
 * Well, what would get him to feel more strongly about Katara than her bringing him back to life?
 * Katara bringing him back to life a second time? Remember that she more or less caused his waking in the first place.

Combustion Man is a double agent for Azula.
Azula, as has been mentioned many times before, plans for every contingency. She could have deliberately planted the idea of hiring an assassin into Zuko's head, because—say it with me now—she is a Manipulative Bastard, and because she has just as much to gain from Aang's death as Zuko does. Azula did a very dangerous and risky thing—she lied to her father outright about Zuko killing Aang—and if Ozai treats his least favored child like less than garbage for a minor infraction, what would Ozai do if he found out Azula, his favorite, tricked him into accepting "lucky to be born" Zuko back into the fold?
 * That's extremely likely, as Combustion Man refused to stop trying to kill the Gaang when the guy that hired him told him to.
 * Word of God says that it was because he figured that he'd get more money from Ozai for killing the Avatar than he'd get from Zuko for not killing the Avatar.
 * In the commentary, Word of God also said he could alternatively have been working for Azula by that point.

Ozai goaded Ursa into killing Azulon under false pretenses.
As theorized here, Azulon may have truly commanded Ozai to give up Zuko to Iroh as a replacement heir. But in a bid for the throne, Ozai deceived his wife, telling her that Azulon had commanded him to kill Zuko. Ursa, fearing for her son, either killed Azulon herself, or somehow aided and abetted the murder. Fleeing into exile, she began to plot Ozai's downfall with the White Lotus Society, aware of her husband's ruthlessness, but still ignorant of the full extent of his machinations. Azula may have only witnessed Ozai "explaining the situation" to Ursa, and came to the same conclusion her mother did (though Azula, contrary to Ursa, reacted with unbounded glee)
 * In "Zuko Alone," Azula eavesdrops on the rest of the meeting between Ozai and Azulon and then goes to taunt Zuko with the news/claim that his death has been ordered. Ursa catches her in the act and drags her off for "a talk." The next we see of Ursa, she's waking Zuko up for a hasty goodbye and then fleeing. It could be that Azula told Ursa the same story she told Zuko, Ursa immediately went to Ozai for confirmation, and he spontaneously decided to go along with it.
 * Meaning that if Ozai did lie to Ursa and trick her into killing Azulon, then Azula was in on it.
 * Which is impossible. Azula went right to mock Zuko and Ursa just happened to overhear. There's no reason Ozai would bet everything on this when spontaneously caught given SHE was the one who proposed killing Azulon. Doesn't work.
 * Or, conversely, Azula just said that Ozai was going to kill Zuko to torment him, and then when Ursa confronted Ozai about it, he sensed an opportunity and ran with it. The fanfiction that proves it.
 * We've seen before that Azula loves nothing more than to make Zuko suffer, so it wouldn't be surprising if she twisted what she'd heard to make it sound worse (or, she possibly outright lied to Zuko). Maybe, after hearing what Azula told her, Ursa went to Ozai with this false information; Ozai, being the fairly opportunistic bastard he is (see the example below theorizing that Ozai killed Iroh's son Lu Ten), saw this as a perfect opportunity to eliminate Azulon and usurp the throne, and thus goaded Ursa into killing Azulon under the pretense that he had ordered Ozai to kill Zuko.

Azulon was actually going to kill Zuko and Ursa killed him to stop it.
Ursa overheard Azulon planning to kill Zuko and Ursa, being the amazing mother she is, decided that she needed to protect her baby at any risk. She then killed Azulon, Ozai found out and had her banished for it. "Your mother did some traitorous things that night..."
 * Given further evidence by the line about how if you mess with a mothers babies, she'll "bite" you. Azulon intended to mess with her kid. She bit.

The Order of the White Lotus is responsible for Zuko's existence.
It's shown in Aang's flashback in "The Storm" that Pai Sho already existed in his time, so the secret organization closely connected to the game could have, too. Now, the Fire Nation executed their genocide of the Air Nomads at the point in the Avatar Cycle when the Avatar was an Airbender, and the logical conclusion would not be that he was frozen in Suspended Animation in an iceberg but that he had been killed. As the years went by without a new Avatar appearing, it must have seemed obvious, according to the first episode Opening Narration, that the cycle had somehow been broken by death while in the Avatar State, and the Avatar wasn't coming back. So without the Great Maintainer of Elemental Balance and Harmony, where would they get someone to "restore balance to the world" during an imperialist war? Mixing the bloodlines of the two who started the war... it made sense to Iroh. No way he gave Zuko that "understanding the other four elements" lecture in "Bitter Work" just to help illustrate how to redirect lightning: "All this four elements talk sounds like Avatar stuff." And Destiny was okay with that plan—Zuko found the actual Avatar when he was sixteen.
 * Which is when the real Avatar is told of his identity anyway.

Lao Beifong will become a Tophaang shipper.
At some point (possibly an Epilogue), Toph's father will stop acting like a modern Overprotective Dad, and start acting like the wealthy provincial businessman in a feudal society that he is. When that happens, he will come to realize that having a daughter in the Avatar's inner circle can only be a good thing for the House of Beifong. And what's the closest into the inner circle a woman traditionally can get?
 * By that time, not only Kataang but also Tokka might well be established as canon, and the issue if the story is pressed to that point would likely be Lao not wanting to settle for a hick Water Tribesman as a son-in-law, even if he is the Avatar's brother-in-law.
 * And also a future head of state... seeing as how when Hakoda passes on, Sokka will be chief of the Southern Water Tribe. Which is just an isolated village of folks now, but in a hypothetical post-Fire-Nation-war scenario will be instead something with 'remarkable growth potential', as they say.
 * Alternative: Now that the Official Couple that involves Aang has been settled, Lao will show himself to be much darker than we thought in an attempt to Murder the Hypotenuse that stands between him and power-by-proxy. This will earn him a dose of Avatary wrath with a side of pissed metalbender, and for that matter one doubts the target would be pleased herself.
 * Even if he is willing to settle for second best and try to set Toph up with the 'hick' above or perhaps that eligible young Fire Lord via this route, he's still hosed even if he somehow finds someone Badass enough to pull it off.
 * Zuko's not eligible—Mai and him are almost certainly formally betrothed. (When she's a fellow noblewoman you don't generally get to set your mistress up with quarters in the royal palace and then openly visit her without chaperones, but its no breach to do so with your fiancee. Its not like Mai was some village girl concubine, after all—her father's a provincial governor, she has a social position and a reputation to maintain.)

The infamous "cactus juice incident" was engineered by Princess Yue in order to spare her beloved Sokka from suffering in the desert.
Many cultures consider hallucinogenic plants to be a gift from whichever deity is associated with the Moon, and the series has made use of genuine Earth folklore before. While everyone else was parched with thirst, exhausted, and cranky, Sokka experienced a euphoric Mushroom Samba that lasted just long enough for the kids to meet up with a tribe of sandbenders who gave them some water and a ride back to more hospitable climes.
 * Alternatively, she did it for the lulz.
 * Why can't it be both?

Fire Lord Ozai killed Iroh's son, Lu Ten.
He had the motive to do so. The means involved hired assasins. And there was a good cover story, since Lu Ten was in battle and it was so easy to pin the blame on Earthbenders. This will culminate in a scene where Ozai tells this to Iroh while in prison, just to see his reaction.
 * Well it's all possible, but I'm sure he could've arranged the assassination for a bit further time, after Iroh had conquered Ba Sing Se, to maximize the profit for the Fire Nation. Perhaps some kind of an "accident" involving one of the last few fighting querilla groups? No, I don't think Ozai was behind this.
 * Ozai asked Azulon to disinherit Iroh because Iroh had not conquered Ba Sing Se. Ozai's strategy would thus be to make Iroh fail in his conquest. With that in mind, Lu Ten died at just the right time.

The Giant Lion Turtle is an aspect of Koh, the face-stealer, using a Xanatos Gambit plan to corrupt Aang.
Aang basically learns Mind Rape powers from him, which seems objectively worse than any other skill used in the show (even blood bending). Each of the former Avatars warn against leaving Ozai alive... they know the price Aang will have to pay to make that happen. Certainly, now that Aang has a surefire way to defeat enemies without killing them, Technical Pacifist turned Knight Templar seems all the more likely...
 * Koh The Ass-Puller. Wow, he's even more evil than we thought.
 * There's fic of that-and it's a great, creepy oneshot-but I can't remember the title. Otherwise I'd link it...
 * Is this it?

Ozai not taking Azula with him to the Earth Kingdom was punishment for lying to him about the Avatar.
Ozai was noticeably angered when he heard that Azula lied about Zuko killing the Avatar, but we never see her in any way punished for this. Ozai probably decided that Azula was too dangerous to turn against (and still too useful to him), but also no longer entirely trustworthy since she would have been willing to lie to him just for the sake of playing mind games with Zuko. He ultimately decides that the best punishment would be to withhold the approval he knows she craves (while at the same time preventing her from doing anything which might draw glory away from him) by not only ordering her to stay behind, but also by granting her a title moments before rendering it redundant.

Ursa was exiled to the Earth Kingdom
...And was hypnotized in the same manner as the Joo Dees. She was given sort of a general mind-wipe under Ozai's orders, and eventually started to regain some of her own personality. She remarried and now lives happily in the middle ring of the city with her husband and children. Zuko recognizes her at one point after he becomes Firelord, but when he sees how happy she is with her new life, he decides not to reconnect with her.
 * ...Alternatively, she's now one of the Ju-Dees...

Ozai never told Zuko where Ursa was
Remember this line from The Dark Knight, "You have nothing, nothing to threaten me with. Nothing to do with all your strength!" That's the position Ozai and Zuko find themselves in at the end of the series. There is absolutely no reason why Ozai would tell Zuko where Ursa was (assuming he knew). And there was every reason not to tell him; the most important of course was simple spite.
 * Of course there are reasons. Torture and mind control or just simple drugs if applied with enough skill and patience can do wonders for extracting information from even the most stubborn subjects.

Ozai didn't have to tell Zuko where Ursa was
Because Zuko becoming the Fire Lord, and welcoming the other nations' representatives with promises of peace, would be big news in the world, and wherever Ursa is, she must have heard about it eventually. Seeing that, she'd have come home to see her son again. Happy Ending for everyone!
 * Unless she's dead or rotting in some forsaken hole/prison without any news of the outside world.

Ozai had Ursa assassinated
She did, after all, outlive her usefulness. That and lying to Zuko that she was still alive might've allowed Ozai to pull Zuko's strings.

Iroh's turning point was the death of Lu Ten, and he was also partly a Chessmaster
Think about it, when Iroh sent a letter to Ursa in Zuko Alone, he jokes about burning the entire city to the ground. It is obvious that he was like his brother during most of his life, without the whole crazy part. Sometime after the death of Lu Ten, Iroh traveled to the spirit world where he gained new found wisdom. After that he joined the White Lotus Society, slowly working his way up the ranks while helping Zuko look for the avatar. He knew Zuko needed to find his own path, and since finding the avatar was next to impossible, he agreed to join Zuko. He then went on secret journey's to different places without the knowledge of Zuko, like the North Pole where he learned the redirection of lightning. Why do you think he was always apprehensive to Zuko when chasing Aang? He never actually wanted Zuko to catch him.
 * It is possible that Iroh was only putting on a face while writing that letter, as when he pretended to kill a dragon as part of his rite of passage, but instead learned awesome fire-bending techniques from it.

Chit Sang betrayed Team Avatar allowing Azula to find them.
When they meet Chit Sang at the Boiling Rock all they know about him is that he is a violent firebender imprisoned in the Fire Nation's highest security prison and they don't ever ask why he's there. The prison has the scum of Fire Nation society, war prisoners and traitors. We know he's not war prisoner since he's a firebender, so the best case scenario is that he's a traitor, but that doesn't even mean he's against the Fire Nation. By the time he arrives at the Northern Air Temple, they had hidden there safely for weeks, but Azula finds them soon after. Azula couldn't have just followed them from the Boiling Rock because she didn't have an airship.
 * This would also explain a number of events in the play from the Ember Island Players that only Team Avatar could have known as Chit Sang could have heard the stories and passed them on. Specifically the line "Katara was the Painted Lady and I think Combustion Man just died."
 * Also, Fridge Horror the town from the Painted Lady was wiped out for hiding the Avatar.
 * How would Chit Sang have told Azula they were at the Air Temple, though? The Gaang's current location is not mentioned in the prison, and the temple is not an easy place to sneak out of without some method of flight/tricky climbing. Azula being Azula, she may have known Zuko had begun his search for Aang at the nearest Air Temple to the Fire Nation - logical enough - and looked for the Gaang there because it was a likely place for them to be staying simply because of Aang's association with the place and its relative security.
 * He could have used a messenger bird. People in the fire nation seem to have a knack for training animals (given the number of beasts they use so adeptly for military purposes, travel etc), so he could have figured out a way to get a bird to carry a message for a least a short distance. Even if the bird didn't get to Azula, as long as somebody found it, that person would pass word along to Azula.

Azula has masterminded everything
The only thing to go wrong was Aang showing up.
 * She didn't plan on it but delt with it anyway, she realized that Aang would win the war, so she sent out both Mai and Ty Lee, her closest friends, to act as moles. She knew she couldn't beat the Gaang, so she faked insanity when she realized Ozai would lose, so she would have a chance for revenge.

Azulon did order Ozai to kill his son, but didn't expect him to go through with it.
It was partially an attempt to teach Ozai a lesson about getting his priorities straight, as well as a Secret Test of Character (possibly even one to determine whether Ozai was worthy of consideration for being named the new heir). He didn't really think that Ozai would go through with it, and would have stopped him had he actually tried.
 * There's a fanfic of it.

One of the purposes of the White Lotus Society is to be available to take down an Avatar if they go insane or wholly corrupt.
Although the known history has examples of lazy and ineffectual Avatars like Kuruk, or harsh and domineering ones like Kyoshi, there aren't any really evil ones that we've heard of. Barring isolated examples like that one village of the descendants of sore losers, its generally accepted by all nations that the Avatar will of course be a benevolent, relatively unselfish force. Now given that the Spirit World selects potential Avatars for having the proper spiritual qualities and that each Avatar has access to the mentorship and advice of all their past selves, the fact remains that they are human beings with free will. So you'd think at least once in history there would have been an Avatar who went entirely corrupt with their power, human nature and the law of averages being what it is. And even one such negative example on the historical record would have colored people's reactions, no? So obviously something is out there to relatively quickly put down Avatars who have gone entirely Sith Lord about the whole business. Now, it could just be that the Garden of Ancient Avatars reaches out and spiritbends the Bending right out of the Avatar gone rogue or something... but then we also have this secret society composed of the greatest Bending masters and martial arts masters of the world, all of them devoted to the same aims of balance and harmony between nations as the Avatar—and yet who deliberately remain apart from the Avatar. Working with him on occasion, when necessary, but never working for him. Now why would they do such a thing unless they're aware of the possibility that they might one day be in conflict with him? And what other force could hope to take down an insane or evil Avatar except a gathering of the greatest Benders and warriors in the world? It all fits.

Zuko really did plan it.
After the Crossroads of Destiny, many fangirls in denial of Zuko's choice claimed it was all part of an elaborate Gambit Roulette. Thing is, they were right. It's just that Zuko is nowhere near as magnificent as his sister, so he completley lost control of it. (Example: Combustion Man.) Also, he didn't let Iroh in on it, because he felt this was something he had to do on his own.

Toph's storylines outside the main plot didn't really happen but are part of her Blind Bandit persona.
She learned metal-bending as part of a mini-retreat with her friends and colleagues, for example.

Smellerbee and Longshot survived by becoming the new Blue Spirit.
They used their newfound Mask Power to escape otherwise certain death, and were never seen again because they were just that stealthy. Based on nothing other then Smellerbee, Longshot, and the Blue Spirit mask all being last seen at Lake Laogai.
 * Even though it was a chibi short and was obviously not canon, it could be argued that Katara ended up dating either Longshot or Smellerbee at the end of School Time Shipping. After all, Longshot would classify as the "strong, silent type" and Smellerbee has passed as a boy before.

Long Feng killed Lu Ten.
Lu Ten was killed in Ba Sing Se. Long Feng is devoted to keeping the war a secret. It's definitely not out of the realm of possibility.
 * I'm pretty sure Long Feng was born after Lu Ten's death.
 * Zuko is shown to be about ten in the flashback to Lu Ten's death. Given that he is sixteen when the series takes place, that would mean Lu Ten only died around six or so years ago. I think Long Feng was definitely around before that.

The Guru was actually working for the Fire Nation.
The path he gave Aang to achieving the Avatar State would have worked, but he knew Aang wouldn't have been able to let go of Katara. It all proved to be unnecessary anyways, which is why you never saw him or his teachings again for the rest of the show, except for a hallucination in Nightmares and Daydreams.

Toph will die shortly after the finale.
Or she will be "lost" to Aang (taken by her parents for good. After all, "The Swamp" shows people who have been lost in some way (and by death for what we saw.) She MUST be made "lost."
 * Doesn't the swamp show Toph in her "fancy" outfit? If that's the case, then maybe it's her "upper-class" persona that was "lost". See a WMG below for a better explanation. XD

Energybending isn't an Ass Pull at all, but a three-season Chekhov's Gun.
Energybending was the first thing Aang did on the show. Remember that giant pillar of light when he broke out of the iceberg? That isn't any other type of bending. It's done by the Avatar State, which is explained as all the Avatar's past lives coming together in one body. But there's no reason that should make a light (other than stylistic purposes, of course)! Word Of God says "few Avatars ever learned Energybending, and fewer still ever used it". So some at least did know about it. The Energybending knowledge gained from these past lives amplify the bending power in the Avatar, resulting in the glow - and the giant pillar of energy in the pilot. This makes Energybending a three-season long Chekhov's Gun, which was only explained in the finale.
 * Aang specifically pulls out a scroll and says something along the lines of, "Hey, look at this weird lion-turtle thing," while the Gaang (minus Toph and Appa) are in the library. Guru Pathik and Iroh also talk at different points about controlling the flow of energy through the body. Those events, along with the green glow in the first episode when he escapes from the iceberg all strongly suggest that the writers knew about energybending from the start. The only problem? They saw the huge amount of speculation fans had from the start about everything and assumed they'd pick up on the extremely subtle foreshadowing... it was too subtle.

Azula killed Fire Lord Azulon in his sleep.
She used her cute little-kidness to get past the guards. Ursa left either because Azula, Ozai, or Azulon (in conjunction with Zuko's grumpiness) "convinced" her that it was in Zuko's best interests for her to leave, or to shift the blame from Azula (who she loved even though she [Azula] was a turtle-duck torturing psychopath). It was only coincidence (orall according to plan) that Azula killed Azulon for political reasons shortly after he implied that Zuko would have bad things happen. (Personally, I never thought Ursa did it, despite, you know, and if Azulon didn't die of being freaking old, it had something to do with Azula. Take note of her all according to plan expression, and she was ten, at the most).

Toph will be formally adopted by Iroh
Before the Grand Finale, (i.e. before it's "cool" for her to be with the AVATAR) her parents have decided that she is a disgrace to their family, and disinherit her. Iroh will find out about that, and officially adopt her into his family. Hell, he might even "adopt" Zuko into his direct lineage as well. Toph won't be able to inherit the throne (nor would any of her decendants, should she have any), unless they married into the royal line. However, Toph would inherit everything of Iroh's, such as titles, land, etc. Following this logic, someday Toph could inherit the title of "Dragon of the West".
 * Not why he is called the "Dragon of the West", it is because he can breath fire, Toph cannot.
 * First, please forgive me for this, but where does it say that that is why Iroh is called "The Dragon of the West"? Second- would you want to argue with Toph and Iroh- if Iroh says that Toph is the new "Dragon of the West", I would shut up and do exactly what he said. XD
 * When he gives his 'demonstration' of fire-breathing to Azula, he first asks whether she knows why he's known as the Dragon of the West - leading to the conclusion that it's down to his use of that ability, not an inherited title. Though it would make for a cool title.
 * The title of "dragon" is given to firebenders who successfully killed a dragon. Iroh was given the title because he "killed" the last dragons. (in actuality, he simply learned from them, and lied about killing them so the fire nation would stop poaching the creatures who gave them the gift of fire.)
 * Well, he did learn firebending from it's source by not actually killing the last dragon, and Toph leanred earthbending from its source- for all we know, Zuko could restructure the name meaning of "Dragon" just for that. XD
 * "Badgermole of the West" is not a very flattering title.
 * No, it's not, but since Iroh earned "Dragon of the West" by actually learning bending straight from dragons, Toph would get the title "Dragon of the West" (inheriting it from Iroh because she's awesome like that), and it would be hand waved as being because she too learned bending straight from the source.
 * Iroh got the title for claiming to kill the Dragons, not for learning from them. It was made up by Sozin as propaganda, so that it would start a chest-pounding match between fire nation generals who wanted the glory of killing dragons and earning Sozin's favor. The title doesn't seem to be passed down because Lu Ten was never referred to as "Dragon". Toph could certainly be adopted by Iroh, but she will not have the title. (unless, of course, she actually killed a dragon, like Ran or Shao, though whether or not the Earth Kingdom would care for or even like such a title is questionable.)
 * Yes, that was the propoganda, however, as Iroh never actually killed a dragon, I was merely suggesting that the name be given new meaning, i.e. the bender learned from the original benders, and as such Toph would actually be able to inherint the name from Iroh because she too learned bending from it's original teachers. (At least, that's what would be said in-universe just so that Toph could be called 'Dragon'.)

Iroh accompanied Zuko on his banishment partially so he wouldn't capture the Avatar.
He encouraged him, taught him stuff, whatever, but if it looked like Zuko was really going to succeed, Iroh would make sure that he didn't.

Ozai lied to Zuko when he implied Ursa was alive.
Think about it. The goal was to keep Zuko there and keep him talking until the fire bending came back "on" at which point, Ozai tried to fill his eldest full of electricity. As he was getting ready to leave before Ozai could do this, he let loose the one thing he knew FOR SURE would keep Zuko there and talking, the fate of his mother. Further implied when Zuko later goes and demands his mother's location from Ozai who says nothing and the screen cuts away.

The Ember Island Players are trying to foment sedition.
Their versions of Ozai and Azula are more laden down with villain tropes than the actual characters themselves. Having the play end with their victory over the wacky gang that the entire rest of the play has been following seems like a hasty cover for their true allegiance, as described here.

Soon after the finale, Sokka rewrites the Ember Island Players' script
He's already tried his hand at painting and been shot down, so he gets the idea to do literature! However, he doesn't quite know how to get started, so he uses something that's already been created and improves it. Plus, y'know, he was kind of there when it all happened.
 * And he's got access to the other side of the story, too.

Sozin's Comet showed up at the same time a huge Air Nomad festival was going on.
That's why Sozin was able to massacre all of the Air 'Nomads' at the same time. They had all gathered in the four temples to celebrate something (my best guess is the Autumnal Equinox).

The Painted Lady couldn't show up until the village cleaned the lake.
This just seems so obvious that I'm surprised no one mentioned it before. Sort of like cleaning the river spirit in Spirited Away or calming down Hei Bei. That would explain why the Painted Lady didn't do anything to protect the village. * So it's like how Captain Planet can't actually fight actual pollution?
 * I always figured she didn't show up in order to help the villagers be able to be self-sufficient, I mean there have to be situations and troubles that will take place for those who go outside the village, I mean if they relied on her too much I guess since she is just that town's legend she can't do anything for them outside that village, and I guess if they ever were caught in a dangerous situation outside their village they'd be just utterly lost without her, I just figured she was preparing them for that, helping them be able to help themselves where she has no spiritual authority over.

Kya had other relatives taken by the Firenation.
In the Southern Raiders, she seemed genuinely surprised that they're going to kill her instead of imprisoning her. It would also explain her lack of family, where as her husband still had a mom. Perhaps her mom was taken by the Firenation, along with any other living relatives.
 * Not necessarily her relatives. It's canon that the Fire Nation repeatedly raided the Southern Water Tribe and abducted water benders. The reason she expected to just be captured is because that's exactly what they'd been doing for the last 100 years.
 * The ability to bend is Genetic, no other members of Kya's family are ever introduced, make of that what you will.
 * Bending is genetic and Kya was a waterbender, which is where Katara got her powers from. Nobody knew what she was because her powers were very weak and/or her parents had taught her to conceal it to protect her from the Fire Nation.

Hawky formed a detective agency with Foo Foo Cuddlypoops.
Because.
 * I thought that went without saying, but their first client was definitely Flopsy.

Chan’s rejection drove Azula into insanity
Before the Beach no one had dared decline a request from Princess Azula, who didn’t know how to handle rejection, especially on a personal level. With Chan’s staunch refusal of Azula’s advances she began doubting her self-image of perfection and control, eventually spiraling into a full-fledged psychological breakdown.
 * I wouldn't go that far, but Azula's ineptitude at normal teenage socialization (with people who didn't know who she was) was probably a contributing factor, along with everything else. She was used to being best at everything, and the situation on the Beach caused her to wonder how much of this was due to her own ability and how much due to being Princess Azula. She even openly admitted to being jealous of Ty Lee, which would normally be way out of character.

Ozai showed Zuko mercy in the Agni Kai.
We all know Ozai is an asshole. We also know he doesn't have trouble killing a twelve year old Aang. The reason he didn't kill Zuko is because he was his son, so he showed him "mercy," and instead burned 1/4 his face.

Zuko had seen his father fight in Agni Kais before.
That's why he was so scared when he saw that his opponent was Ozai. Agni Kais aren't always to the death but let's be honest, would Ozai really show mercy to his opponents? Zuko knew his dad tends to kill people and was terrified that he'd end up like they did. Like Ozai said, Zuko was "lucky to be born."

After being trapped in the library, Professor Zei became Wan Shi Tong's immortal servant.
So it doesn't end so badly for him after all. He gets to do what he loves best, and he's made peace with Wan Shi Tong.
 * He was the only one who was there for knowledge itself, rather than looking to use it for violent ends. Wan Shi Tong also appreciated the professor's gift the most, and may also acknowledge the dedication it would take for a mortal human to willingly trap himself eternally in such a place purely for the information it held. As the spirit is dedicated to the collection for and preservation of his library, and holds animosity only towards those who would abuse it - not mortals in general - this theory would make sense; he found a kindred spirit, as it were.

Cabbage Man was turned into a cabbage.
In the episode The Puppetmaster, if you look closely at the cabbage Hama pulls out of her grocery bag (near the beginning of the episode), it looks like an old man's face. And if I recall correctly, Cabbage Man is not seen again. Make of that what you will.

In fifty years or so, the series' surviving Extraordinarily Empowered Girls will form their own secret club.
By the time of Legend of Korra, a group of tea-drinking old ladies will control the fates of nations.

Toph Beifong succeeds Bumi as the ruler of Omashu.
Unlike all the other monarchies/chiefdoms, the heir to the throne of Omashu is determined not by heredity but by earthbending prowess, in honor of their founder, Oma, the first earthbender. After being ruled for almost a century by the most powerful earthbender in the world, the people would only accept the greatest earthbender of all time as his successor. Also, Queen Toph? Rule of Cool must apply here.

Sokka's sword and/or boomerang will make a reappearance in The Legend of Korra
Some unknown warrior type main character will find it and it will be a nice little gag about how the two weapons were along for the ride of adventures of the Avatar.

What really happened between Jin Wei and Wei Jin.
Obviously, Aang's lie is just plain ridiculous so here are my possible theories on what really happened. Anybody is free to add their theories.
 * User:Nightelf 37: Jin Wei tripped up, and Wei Jin then entered the scene. He knew about the Gan Jin's Redemption ritual, but not much. He knew that the orb had to be bought to the gate, but he didn't know it had to be a Gan Jin who had to carry the orb. Wei Jin took the orb, with the intent of completing the ritual. Jin Wei was conscious enough to see Wei Jin do this, and thought he was stealing the orb. The Gan Jin guards at the gate, thought so, too. They were posted there to arrest anyone who dared interrupt the ritual. Since Wei Jin didn't know much about the ritual, he was mistaken for stealing the orb and was arrested for that 'crime'.
 * User:Nightelf 37: Jin Wei was hurt and asked Wei Jin to help finish the process of the sacred orb. He was mortally wounded and didn't survive. When Wei Jin reached the final destination, he was wrongly accused and imprisoned for killing Jin Wei and stealing the orb because Jin Wei was not there to back up his story.

The ENEMY that haunted Azula into madness is not the "All Powerful" Avatar or any inner demon. Its Ty Lee. Not even Mai counts.

 * Mai is like Tylee, a skilled non-Bender but one Azula has seems to have confidence in defeating in battle. The thought of Ty Lee winning a fight, or even daring to fight her, is absurd to Azula, yet it happened, and Azula never saw it coming.
 * In ALL of her battles, Azula has never been beaten. Even agains 5 to 1 odds VS Iroh, Zuko, Aang, Toph and Katara, she held her own and withdrew unscratched. For someone to take away all of her powers and rendering her helpless was a form of treachery that shook Auzla to the core. Confronted with an enemy she didn't see coming, one that could have killed her at whim, wearing the mask of a loyal servant and friend, THAT is Azula's nightmare. Her coronation was kept to bare minimum because of Ty Lee's effect.

The meteor Sokka's sword is made of was a gift from Yue.
Not all that out-there. It was just too convenient.

Getting out of The Cave of Two Lovers is not based on of The Power of Love, but The Power of Trust.
Specifically, if one person suggests blowing out the torches to let the crystals light the way, the other person has to trust the first person enough to know that they aren't trying to ditch the first person in the dark.

King Kuei is an evil overlord who uses obfuscating stupidity to fool everyone.
Kuei is actually the mastermind behind everything in Ba Sing Se. He knowingly plays into the cliched trope of the evil chancellor misleading a king, making everyone think Long Feng is behind the city's oppression when it's really just him. That way he stays popular and, by playing the fool, can more easily manipulate those around him. He was testing the Gaang to see how they could handle the problem of Long Feng, and when they finally bested him Kuei decided to finally back the Black Sun invasion. Long Feng takes the fall because he's a true patriot.

Then Azula shows up. Kuei decides to conduct another test, this time to see how the Avatar confronts a direct threat. The added bonus is to determine if Zuko and Iroh, who the Dai Li have been monitoring all the while, can be brought on-board for the invasion—someone needs to replace Ozai as Firelord, after all, and perhaps the Avatar can score a firebending teacher while he's at it. Only everything goes pear-shaped when Azula manages to "kill" Aang. Because they think the Avatar's dead at first, the Dai Li fall back on "Plan B": surrendering the city to the Fire Nation to avoid its annihilation during Sozin's Comet while making themselves useful to Azula. That way the Dai Li can get the score on the Fire Nation government's inner workings.

Kuei ditches the Gaang to secretly reunite with Ba Sing Se's underground resistance, led by loyalist army and Dai Li officers, and uses the city's occupation as an excuse to eventually conduct a purge of true collaborators and inconvenient official/nobles using the cover of the Sozin's Comet liberation. The Dai Li that join Azula in the Fire Nation try to covertly protect the Avatar on the Day of Black Sun by orchestrating the big brawl so that Azula can't get such great line-of-sight for lightning again (all those thick stone pillars thrown around, blocking things).

At the eventual peace conference, Fire Lord Zuko, who's been fed these stories by his friends of the man-child Earth King and his pet bear, never realizes that he's sitting across the negotiating table from a canny operator who makes his sister Azula look like a chump. The Harmony Restoration Movement? It's nothing less than Kuei getting Zuko to ethnically cleanse the colonies for him, stirring up discord within the Fire Nation while leaving the Earth Kingdom with clean hands.

The People of Ba Sing Se Will Not be Grateful For Iroh's Liberation
Just based on what went down at the end of the episode Zuko Alone. Plus, occupied Ba Sing Se has seen more of the Fire Nation's brutality than that village, much of it coming from Iroh's own niece. He's still hanging out in his tea shop at the end of the finale, but as soon as the city gets back on its feet he'll be turfed out.

Iroh wanted Zuko to become the Fire Lord right from the beginning, because he realized Ozai needed to be replaced.
Iroh had for long doubted Fire Nation's justification for the war, and after his son died in Ba Sing Se, he finally saw that the war needed to end. However, knowing his brother, he had severe doubts whether Ozai would cease the war before the other nations were utterly destroyed. When Ozai challenged his own, barely teenaged son to an Agni Kai because of a few misspoken words, and subsequently burned his face and banished him, Iroh finally realized that his brother was an extremely cruel and ruthless man, and that he needed to be replaced as the Fire Lord, if the war was ever to end peacefully. However, Iroh himself didn't want to become the next Fire Lord, he was too old and too mellow for that position, plus when he would die, the question of his successor would rise again. That's why the only alternative would be one of Ozai's kids. Now, Azula had already proven to be unstable and almost as cruel as her father, so she wouldn't make a much better Fire Lord. But in Zuko Iroh saw the potential to become a benevolent and just king; after all, the inappropriately timed comment that lead to the Agni Kai had risen from Zuko's sympathy for the common soldier.

So, when Zuko was banished, Iroh decided to follow him, seeing this as the perfect opportunity to help Zuko grow up wiser. Iroh knew that you can't force anyone to become a righteous person, but what he could do was try to make Zuko find his own way, to see if he could emerge from under the shadow of his father and become a better man. So Iroh stayed with Zuko, questioning the beliefs Zuko had grown up with as the son of the Fire Lord, and trying to help him find a direction that Iroh knew would be better for both Zuko and for the Fire Nation. When Zuko chose his father's side in ”The Crossroads of Destiny”, Iroh of course was extremely disappointed, but in the end Zuko came to realize the evil ways of Ozai, and everything worked out just as Iroh had hoped it would.

Iroh original plan probably didn't include the Avatar, because like everyone else, he thought the Avatar was lost. Most likely Iroh didn't know about Ozai's plans for the return of Sozin's Comet, so he thought he had plenty of time to help Zuko grow into a master firebender, who could then challenge Ozai. He probably would have at some point taken Zuko to meet the Sun Warriors and the dragons, so he could learn true firebending. Of course, when the Avatar did show up, Iroh realized that Aang would be of help in his overall plan to get rid of Ozai and replace him with Zuko.
 * The most obvious proof for this theory can be found in the episode "The Earth King", where Zuko falls into his Angst Coma. Iroh tells Zuko that he's going through a metamorphosis, and that he's going to turn into the "beautiful prince you were always meant to be". The metamorphosis was caused by Zuko's growing conscience, and Iroh clearly wishes it would make Zuko choose the path of peace, instead of following the ways of his father and sister, so Iroh using the world "always" would imply he has for a long time known (or at least hoped for) that Zuko would eventually choose this path.