Okay at this point here are the things I think are possible for today and my reasons for them.
1. Shulk - Arguably the "Boring" choice. This is most likely to me because we do pretty much know he's coming, but because of that I don't want to see him. The retweet places him above Chrom and Chorus too.
2. Ridley - He's been teased for ages, it's about time we got the low-down on what's happening. A single one-off presentation seems like the perfect opportunity to show up what Ridley is like and how he relates to Pyrosphere.
3. Mewtwo - A serious fan-favourite, three minutes is a long-ass trailer so this would give them time enough to cover the possible three different versions of him and how they work together.
4. Rayman - Unlikely, but given the trophy today I had to put him here. Given that Sakurai specifically mentioned that the trophy was made by Ubisoft for this purpose I can't imagine him being anything more. However I would be very happy to see him.
5. Assassin's Creed Rep - I can already hear you all screaming at me but hear me out. Given the Rayman trophy we can assume Ubisoft has some level of involvement with this game. It's also unlikely that Rayman himself is playable so we can look towards other potential reps to Ubisoft. One that springs to mind is Assassin's Creed which is probably their biggest selling franchise. It has had two DS games (Both of which were originally DS exclusive) and the last two main titles have been released for Wii U and presumably the unrevealed but leaked Assassin's Creed: Comet. This is very similar to the level of coverage Solid Snake had in Brawl and an Assassin would cover the more mature audience that Snake covered in Brawl. One final piece of evidence you can look to is Soulcalibur V, which like SSB4 is also a Namco game and did feature Ezio as the guest star. This proves that Ubisoft is happy for Namco to use the character if they wish.