I doubt there will be any completely new characters that have been never seen before. Not in a season/saga finale were any completely new character would just get in the way and steal screentime from a cast that is already as big as it is.
Actually, to it seems like there's no way to
not in introduce new characters. I go more in detail
here, but long story short, Nomura said this:
Nomura said:
The process of the wielders coming together will be the main storyline, but one of the highlights of the climax is going to be who is part of the seven lights and thirteen darkness when they get to the final battle. Would it be as everyone expected, or will there be unexpected members added to the mix? That is something to look forward to.
Which means that who are the seven and thirteen will only be revealed in the final battle. Which means
not even the current members of Organization XIII are all confirmed.
For "surprise members", to me that seems to mean that people we wouldn't expect to redempt, like Young Xehanort or Vanitas for example, would become a light, and people who we wouldn't expect to side with Xehanort being a darkness. While we have some characters in the "to be redempted" list (which I think would be redempted before the final battle), we don't have a "to be corrupted" list, and it wouldn't make sense for Nomura to suddenly make any of the heroes into villains. The best I could come up with would be norting the Lingering Will since that thing barely has a personality of its own and currently doesn't have anything that can make the players sympathize with him. So the only way around it would be introducing some new ambiguous characters, or characters pretending to be good, who at the end reveal to be evil.
Plus, even ignoring my "multiple treasons" theory, we don't have enough evil characters to fit the unknown members of the Organization. We have seven empty seats, six of them filled by unrevealed people. Terra is implied to be somewhere, so that goes one. Vanitas and Riku Replica are often theorized to be among them, which if correct would still leave us three, although I'm unsure on whether they would bring back the Riku Replica.
Marluxia and Larxene were traitors, and ordered to be eliminated, therefore we can scratch them. Ienzo and the others weren't picked up, so we can scratch them too. Demyx didn't show norting signs, and is the weakest member of the group, so he is almost scratched.
Luxord is... pretty ambiguous. He has yellow eyes, but no pointy ears, and Nomura basically denied him a backstory so far, like the other three members, so I'm not sure if he's that important to the story. But if he is, he still leaves two seats. What would you fill the other two seats with, and also the empty one for that matter? I doubt a Disney character would be a Xehanort, and I can't think of any other known non-original evil character who is not listed here. The best I can come up with is Xehanort creating a Data-Xehanort, but that only fills one seat.
Unless the claims trace back to a credible source, I wouldn't stake my trust on Wikipedia. Anyone can edit the articles and cases of false information and vandalism have happened in the past.
You do realize that Wikipedia, and Wikis as well, have things called "citations", don't you? They are there
exactly so that people can check the info. "Anyone can edit" is not a good argument for whether a site is trustable or not, provided there are citations to reliable sources (and you can verify the claims), the site can be trustworthy. Also, false info and vandalism are anomalies, and they are one of the reasons there are admins to maintain the order.
Working under the assumption that the GoL will all be Keyblade wielders, I don't suppose there will be any new characters among the ranks. Even with the current characters, there's a surplus of Keyblade wielders who are likely candidates to be guardians; we have the main trio, the BbS trio, Mickey, Riku, Lea, and potentially Roxas and Xion. That makes 11 of them, which can buffer up to 4 missing heads before someone else has to fill in.
Nomura said in an interview that the guardians don't need to be Keyblade wielders.
Besides, I'm not quite sure if introducing an entirely new character just for this purpose is a good move on the writers' part. They would have to create a whole backstory for him / her to explain about various things, especially where he / she fits in all this affair and why he / she should be one of the GoL. It's not impossible, but it's just that it might probably not be a wise decision.
How about simply "I don't want darkness to overtake the world"? Backstories can be interesting, but they don't need to be very detailed for the character to fit. Demyx and Luxord fitted well in KHII's plot and we know very little about them, in fact, their backstories for how they fit there were simply "I lost my heart and want it back". Same thing with Marluxia and Larxene.
Even the Leon and others had simple backstories: "I survived my world's (incomplete) destruction, and I'm here to help prevent the same thing from happening to the entire universe". So, again, we don't need very big or detailed backstories for the characters to work.