Xehanort because he is the main villain of the series apparently, unless if Nomura wanted to show his good side since the next game is going to focus on his past....
or maybe the UEM since the video and his ever-so-freaky appearance clearly show his villainy. If Nomura just made UEM the main character, or hero, of the prequel then it just wouldn't right...UEM doesn't seem to be a hero, obviously.
As for the main character, I think it will be either Sora, Terra or Ven...although the other two seem likely, I put up Ven since he is somehow connected to Sora by name.
Not exactly but I think that UEM is most likely Xehanort because they say that Terra is Xehanort's avenger and Terra does show anger and hate towards the UEM in the "Birth By Sleep" video, mostly through his fighting and his expression at the end of the video...just a thought.