it cant be xeanort he is an antagonist how would he have a story
They could pull a Darth Vader. Have the main antagonist realize the error of his ways and commit to fixing it, but I doubt that.
The way Nomura says, "now is
also the story of one other person." Might be translation, but it sounds very fishy. Like he means more than he said. To me it sounds like Sora will still play a role in the Mystery Series, just he won't be the main protagonist. And he seems to emphasize that it is the story of ONE person. So, to me, personally, it rules out Riku and any of TAV.
But he says that the the Mystery Game will "connect the series". So, the big question is if he means connecting BBS to KH, as in connecting THE series, but I was left with the implication that BBS was already pretty well intertwined OR does be mean connecting KH to the new Series.
I think that we have met the new Protagonist, but he/she did not play a major role and was overlooked the first time we met them. I would say that the Protagonist will be introduced in KH3, BUT Nomura said that the Mystery game will come out first (and the Mystery game will probably be the launching point for the new Series). We can probably count on the Protagonist being a Square-Enix original character, nobody from FF or Disney, at the very least (like SRK and TAV were).