I feel like with Ubisoft, it's not a matter of if an Assassin's Creed character will get in, but which assassin will be chosen. Between Ubisoft's openness for putting their characters, especially assassins, in other things - see FFXIII-2, TF2, MGS4, and SCV - and the way the AC series is often given a stronger push on the PS3, I'd honestly be a little surprised if an assassin doesn't get in.
Which one, though?
Altair:
+ The first assassin, arguably the more classic design.
- The first game is pretty much universally agreed to be the weakest.
Ezio:
+ The most popular, since he's been in the most games and the best games thus far.
+ Has had three games to build up a potential move list.
- Has been in so much since he premiered in 2009. People might be a little bit tired of him.
Connor:
+ Will be the most recent main character when All-Stars releases.
- His tools and fighting style seem to be different from the more iconic stuff in the series. No sword, no dual hidden blade; instead, the tomahawk, bow, and gun.
- Depending on when they finalized the character selection, and how open Ubisoft wanted to be about the development of AC3, they may not have even known about Connor early enough to get him in.
I'd say Ezio and Connor have about an equal chance, depending on the circumstances. I'm kinda hoping for Connor, personally.
If they decide to go for Ezio, I bet it wouldn't be a problem for them to make Altaïr an alternative skin for the same character. They don't really differ too much in the AC games, and would probably differ even less in this game.