This could be great. Sony should be able to score some solid 3rd party characters, too. And I suspect the game will be T rated -- so some blood and bruises but nothing gorey.
Lara Croft vs Nathan Drake? Hell yes.
We know Ubisoft is open to having Ezio in fighting games, and Sony has marketed a lot with them on Assassin's Creed, so I think we could expect something from them.
Not sure about Final Fantasy since Square makes the Dissidia games, but Lighting is probably the most likely candidate.
Snake or something Metal Gear is obvious.
Big Daddy or one of the characters from Bioshock Infinite, perhaps.
Ryu from Ninja Gaiden
And pretty much any game character that will have a presence in 2012.
Really, there are a ton of possibilities. I think having the game aimed at a slightly older demographic is an asset... it opens up a lot of crossover potential.