Resident Evil 6/WMG

Leon and Sherry will reunite during some point in the game
Because the developers said the stories will intertwine in some parts.
 * Confirmed

The third protagonist is HUNK
HUNK is the one with immunity in his blood. This explains the rumour the helicopter pilot says at the end of Resident Evil 3 about how he can't die and that he's the only one to survive in all his missions. Even if it is a successful job, the rest of his team eventually succumbs to the virus while he's immune. In the beginning, he probably thought he was just deathly lucky but only now does he realize he has virus immunity. HUNK, now a standalone mercenary after Umbrella fell, is then hired as a bodyguard since he has BOW experience.

In addition, the third protagonist states the line: "much better off as a mercenary" possibly referring to his past job as an Umbrella mercenary/agent. This line could also be a nod to the audience. HUNK is now an actual protagonist but he's wondering if he's better off just being in the Mercenary side games.
 * To support this, look at this image of HUNK unmasked and look at this image of the new guy. They look quite similar, with the usual difference in graphics, of course.
 * In addition, there's this image of HUNK unmasked. Though, the hair is a different colour.
 * Jossed. The 3rd Protagonist's name is Jake Mueller, Wesker's "son."
 * For all we know about HUNK, he may in fact be Jake Mueller...

The third protagonist, the mercenary, is Steve Burnside
The mercenary has short, red hair, which may be Steve shaving his head out of practicality for his job. He is also unnaturally swift and packs a powerful punch, hinting at virus-enhanced abilities. It's mentioned that the world needs his blood, and it's implied to be expensive; it could be his resistance to the T-Veronica virus, and his antibodies are important in making a cure for the hodgepodge of viruses on the loose, or it could be the virus itself and many factions desire the virus for their own agenda. In Code Veronica, Wesker mentioned that reviving Steve was a possibility, and he may have actually done it.

His dialogue and attitude seem very much Steve as well, more so than the enigmatic and generally quiet HUNK. It immediately sounded like Steve, were Steve to grow up a little but maintain the same ego and style of speech from Code Veronica.
 * One of the lines the third protagonist says is, "...much better off as a mercenary". If he's referring to himself, then it can't be Steve since he's never been a mercenary.
 * He's already a mercenary before the events of the game though. It could be him thinking what was supposed to be A Simple Job before B.O.W.s came and changed everything, especially when one of them seems to be specifically hunting him down.
 * Furthermore, we have no idea what Steve ending up doing after Code Veronica. Given his experience with B.O.W.s from that game, as well as the T-Veronica virus inside of him, which is consistent with the "needs my blood" line, he very well could have gone into work fighting terrorists for the highest bidder.
 * In Darkside Chronicles, Wesker mentions that he takes a sample from Steve's corpse and gives it to Javier subtlely hinting that he's not keeping Steve alive or in stasis. Unless Wesker only figured out how to bring him back to life afterwards.
 * That's what his scar is from. It's what happened to the virus collecting incision after either being returned or released from stasis.
 * Wesker saved Jill by preserving her body in cryostasis. That prolonged exposure to the virus from Resident evil 3 and her term in cryostasis allowed her to develop antibodies to all viruses. It's possible that he always had Steve in cryostasis for testing purposes and this helped him develop virus antibodies. The mercenary also displays the same superhuman acrobatics as Resident Evil 5 Jill had.
 * And, if the cryostasis process wasn't as developed when he got Steve, it could be the reason that he's bald. Wouldn't be the first time it's messed with a subject's hair. It's just been tweaked so it just damages hair enough to lose its pigment instead of damaging it enough to fall out.
 * Except that he has visible head stubble, indicating that he shaves it rather than him being naturally bald. And on a meta level, it seems like Capcom did that on purpose just to make it harder for us to figure out who he is.
 * Jossed. The 3rd Protagonist's name is Jake Mueller, Wesker's "son."

The mercenary's blonde companion is Sherry Birkin
Like Steve, Sherry is also a virus infectee, specifically the G-Virus. The bioterrorists that killed the President of the United States may have gone after Sherry now that her protection is in chaos, and the best they could do is hire Steve to rescue her. That, or she Took a Level In Badass and became an Action Girl, and she now works for the U.S. Governement and is assisting Steve, if her unhesitant stabbing the mercenary with a syringe is any indication. There are also several visual indicators: She wears a blue scarf, which brings to mind Sherry's blue uniform from Resident Evil 2, has very bright, blonde hair, and is very young-looking, which fits as Sherry would be 22 as of RE6.
 * Confirmed.
 * "We have exclusive information about the blonde woman in the trailer - and she's not who you think she is." Apparently IGN is unfamiliar with WMG.
 * Confirmed via recent trailer.

Leon's companion is Sherry Birkin
In Resident Evil 2, Sherry is shown to have a somewhat matching shade of dirty blonde hair, and the woman who claims to be responsible for the events in Resident Evil 6 looks a bit like Sherry's mother, Annette. Sherry was rather intelligent for her age, and no doubt could have gone into a scientific field that dealt with counter-terrorism to try and atone for what her parents had done. This could in some way come back around and cause her to be responsible for an outbreak of her own.
 * Jossed. Leon's companion is a new character: A government agent named Helena Harper. As mentioned above, Sherry is the mercenary's companion.

The third protagonist is Alex Wesker
As discussed in the Steve WMG, this man appears to have enhanced speed and/or strength, hinting at virus-related powers similiar to those famously displayed by Albert Wesker. We know Alex was administered the same virus, which would explain this. We also know Alex was doing an incredible amount of work researching ways for Spencer to become immortal during the events before RE5 (from the "Lost In Nightmares" DLC). It is entirely possible he discovered a cure to the T-Virus (or whatever virus threatens the world) during his experiments and absconded with it to make money (hence his desire to sell his blood, and the reason he dropped out of contact with Spencer). Alternatively, it is possible the experimental Wesker virus itself secretely holds the cure to T-Virus infection, and this is why neither this new protangonist nor Albert Wesker were ever infected, despite the latter in particular being around zombie-infested areas on numerous occasions.
 * Wesker? As a protagonist?
 * We've never seen him before. Why automatically assume he's evil at all?
 * The notes from Resident Evil 5 and additional DLC pretty much spell out that Alex is, with mentions of experiments costing "thousands upon thousands" of lives and a quest for Immortality, which is a pretty typical villain goal especially if you are willing to kill a whole bunch of people to get it.
 * True. However, he could still be a possible villian protagonist, or reformed (after a fashion- the mercenary's clearly a jerk). There's also the information that whoever the mercenary is, he's "related" to one of the main RE characters in some form. Assuming related is not being taken literally (as in he's not Leon's long lost brother or something), Alex Wesker most certainly is this to Albert.
 * It's a small thing, but notice that his outfit is all black and his sleeves are 3/4 length, which is certainly reminiscent of some of Albert Wesker's costumes.
 * Jossed. His name is Jake Mueller, Wesker's "son."
 * Because Villains Never Lie apparently.

This won't be a "simple" Shooter, but...
... Leon's part will be more "Survival Shooter" (like Resident Evil 4 or Dead Space), Chris' part will be a straight Third-Person Shooter (Like Gears of War, I mean: Look at Chris taking cover), and the "Wesker child"-part will be Beat'Em Up (like God Hand, MadWorld or Asura's Wrath).

Alex Wesker is a Girl
And she is the "bitch" Chris Redfield was shouting about in the trailer. You might also hear his radio saying that he is after the "Ace of Spades", which presumably means much the same it did in the Iraq War. So we can assume Chris and his team are in China to capture a major wanted criminal and may not have been expecting the Plagas outbreak. Assuming Spencer really is dead (or at least that the good guys still think he is dead), and so are Albert and all the other Umbrella bigwigs, it's either a totally new character or more likely Alex Wesker.
 * Alex was specifically noted to be a "he" in Spencer's reports from 5.
 * In the japanese version, Alex Wesker is referred to as a male.

The G-virus might be involved.
The G-virus is still going around if Degeneration and the t+G experimental virus in Dead Aim are anything to go by. We know that Umbrella did some experiments and analysis on the G-virus after HUNK delivered them a sample in 2, but we don't quite know what else they did beside combining it with the t-virus. If the Progenitor virus can eventually be refined into advanced strains like Uroboros, who knows what else Umbrella could've devised before they folded. The Javo looks like it could be an evolution of the single eye G-virus-infected hosts sport. Additionally, Sherry Birkin had a G-embryo implanted in her during 2. While she was given the G-vaccine soon after, the vaccine only suppresses the virus's effects, and doesn't actually eliminate it (which could possibly explain her newfound athleticism).
 * Her newfound athleticism might also be a result of, y'know, not being twelve years old anymore. Most of the women in this series are quite athletic.
 * Given how this is completely plausable human acrobatic in the Resident Evil universe, I wouldn't say Sherry's athletcism is abnormal.

Ashley Graham between 4 and 6
Ashley graham is confirmed to be in the game and is in the Mercenaries section as the sidekick. Since RE 6 is set during 2008, Leon probably taught her self defence and gun safety in order to allow her to protect herself (and make the Secret Service's job easier). She is also seen using a syringe on the Merc, suggesting that she became a medic or doctor, probably joined BSAA. Since co-op is confirmed, Player 2 will be in control of her, so she definitely Took a Level In Badass. However, I have no idea why she's with the Merc, but since he's a cure to a virus, she was probably taking his blood to help create a sure. However...
 * That's not Ashley, that's Sherry Birkin. Plus, it seems like the term "ten years" is more of a rough catch-all time period, seeing as this can't take place in 2008 if it's set after 5 (which took place in 2009), not to mention that the the official Xbox Marketplace description states that it is set in 2013. Either the story writers goofed with the dates, or this really is an interquel (which seems unlikely, seeing as there was no hint of a global bioterrorism outbreak in 5).
 * Who's to say that 6 DOESN'T take place before 5?
 * I did note that (hence the words "or this really is an interquel"), I just said it was unlikely (from my point of view). We don't know the full scope of things yet, so there's no need to get riled up. The WMG currently at the bottom of the page which mentions Revelations does bring up an interesting point...
 * RE6 does indeed take place in 2013.

What is Ashley doing back in Not-Spain?
At several points in the trailer, you can clearly see certain landmarks and locations from 4, such as Saddler's mansion. Unless she was captured again, I doubt she'll be there willingly. Perhaps she got herself captured in order to investigate, or has an internal tracking device now?

The game has 3 different modes
You can play the survival horror (Leon) game, the third person action (Chris) or action horror (Jake) game at any time, switching when you're stuck. Go further in one mode, unlock a gun, use it in another.
 * Kind of sort of Jossed. They aren't modes per se, but a different type of survival horror was in mind for each segment. Leon's is called "Gothic horror" said to resemble earlier iterations of the series. Chris's segments were called "military horror" likely similar to RE5. Jake's section involves being chased by an unrelenting monster, like Nemesis. They aren't different story modes, though, simply different campaigns.

The "Bitch" Chris refers to is Alexia Ashford, the Veronica virus returns, and the third protagonist is Steve
The Veronica virus is the only virus that requires direct injection in order to spread it. That explains why a syringe bomb was created. Alexia Ashford isn't the main protagonist but someone has took her research on the veronica virus and has expanded it. This explains why the protagonists seem to be fighting able-thinking, gun-using humans; the veronica virus allows mind control for the host. If it really is the veronica virus, then it fits in that the protagonist is Steve who is now immune thanks to experimenting and is familiar with this supposedly new strain of virus.
 * Double Jossed. The "bitch" seems to be a new villain, and the third protagonist is Jake Mueller, Wesker's "son."

The game is split by protagonist AND by the type of virus that takes the spotlight
Leon's gameplay seems to be more focused on generic T-Virus zombies with the occasional unique zombie. Chris's gameplay appears to be back in the Resident Evil 4 setting and with plagas. The third protagonist appears to be up against the more fantastic-looking G-Virus monsters. Perhaps the final boss is an antagonist who has taken on all the different viruses into their body.
 * Chris's section actually takes place in China, but hey, technicalities. Seeing as this is a series of random, global viral outbreaks and several viruses (the t-virus, G-virus, Progenitor virus, and T-Veronica virus, among others) have passed hands quite frequently, it's very well possible that we're looking at various strands being active at the same time.
 * In the latter half of the trailer, you see Chris's team in a village-like area with a castle that resembles Saddler's in the background.

The "Bitch" Chris is referring to is
At the epilogue montage in Resident Evil Revelations, it's revealed that.
 * Seemingly Jossed. They villainess seems to be a new character.

There will be a reference to Fong Ling
With the use of multiple characters in this story there is a chance that they might throw out references to the other non-main series games. If this is the case then theres a good chance that the reference will be about Fong Ling. Part of the game is set in China and she works for it's government, plus she has experiences in these matters.

The identity of Chris's partner...
Is Ark Thompson. According to Resident Evil 0, the game is canon. The characters do, as far as the graphics Survivor show, look similar. Plus it'd be quite the fandom nod if he played the calm and collected to Chris's emotional self as seen in the trailer.
 * According to the RE Wiki, it's a guy named Piers Nivans. He apparently showed up in a manga set before 6 (called Biohazard Marhawa Desire) which deals with the viral outbreak in Asia.

Something bad will happen to Jill
Jill has immunity to viruses as stated in Resident Evil 5 so she'd be prime candidate to help fight off the outbreak. The third protagonist is implied to have immunity and is hired over the trustworthy former BSAA agent. Given her personality, she doesn't seem to be the type to take a break from helping Chris unless after being saved her body was too weak. She is either...
 * Dead prior to the game, possibly killed by the antagonist giving Chris a pretty radical personal vendetta
 * The antagonist, perhaps what happened in Resident evil 5 is still having an effect on her
 * A specimen being used for study by either side of the battle, given her immunity

Chris and Jill got together between the last game and this one

 * They may have even gotten married - the reasons Jill may not be involved in this mission is either because she's still recovering from the events of the last game and is on leave. However, if she and Chris are together, she could be pregnant with Chris's child and thus on maternity leave.
 * It's also possible that they're simply in different areas, seeing as it's a global crisis this time around and Revelations already shows that Chris and Jill are able to work competently with other BSAA members. BSAA might've just split up their best members and sent them separately to the biggest viral hotspots on Earth.

The villainess seen in the recent trailer is Ada.
She seems to have a red color scheme to her clothes as well as having dark hair and somewhat Asian features. It would also explain Leon's desire to want her alive over Chris who simply wants her dead. Ada's mysterious connections at the end of RE4 were never revealed. It could be we're finding out about that now.
 * Confirmed by the E3 2012 trailer. She's working for Neo-Umbrella.
 * Or at least Ada is in the game, while an entirely different character may be framing her.

Claire Redfield will make an appearance.
True, there's no info that even suggests she'll be in Resident Evil 6, but since two out of three main characters from Resident Evil 2 are in this game, there's a good chance that she'll either play a part in the plot, or at the very least make a cameo (maybe showing off the red jacket Claire gave Sherry). It's obvious Sherry was very close to Claire at the end of RE2, and it would be a great opportunity for a reunion. Who knows, maybe this time Sherry will be the one rescuing Claire in a TerraSave aid/evac gone wrong or whatnot.

Wesker is back and he's pissed.
Yeah he died in Resident Evil 5. This is Capcom and Wesker's first death. Dr. Wily, Sigma and M. Bison have walked off stuff worse than this. Don't be surprised when you find out that Wesker infused Ada Wong with his DNA and the C-Virus mutates her into Wesker.

Sherry will become a Tragic Monster.
Remember back in Resident Evil 2 (as well as Darkside Chronicles) where William Birkin mutated into 'G'? Chances are, poor Sherry may end up suffering from the same fate as her father.
 * Unless I'm remembering things incorrectly, the G-vaccine only suppressed the G-mutation from the G-embryo placed inside Sherry back in 2. Given the heavy focus on genetic experimentation in this series, there's probably some way to induce the mutation once more to a stronger effect that could somehow reverse the effects of the G-vaccine.

The leaked reveal that ...is/was this game's "Jill is the Birdwoman" twist.
Building off the WMG a few posts up, and assuming the leak is accurate about  I'm guessing it'll be treated the same way, i.e.: (1)Trailers imply. (2) Player/heroes go through 75% of the game thinking as much. (3) Dramatic reveal with "Ba-ba-bum" included. (4) Kill  (5) Heroes get to da choppa and fly off into the sunset.
 * Why? Because even though this'll probably be Jossed, right now I honestly can't think of another reason to  as the villain.

Sherry and Jake will develop a mutual case of Rescue Romance.
Both are captured by a bad guy at least once in the game, and while Jake does seem to be the more powerful one, Sherry is responsible for his safety and helps Leon free Jake from at least one claw monster. After repeatedly saving one another, Jake stops talking about how much more money BO Ws will cost and just start truing to hang close to Sherry.