Maybe I'm just reading the title differently that everyone else, but when I think should that means there is at least some probability/chance of it occurring. Many people have said Gears 2, but if I recall correctly the deal with Epic was for at least 2 games then they'll renegotiate, thus zero chance.
In my opinion the game with the highest chance is still Mass Effect due to the acquisition by EA and lack of public information as to the specifics of the deal with Microsoft.
Of course people may be interpreting the question as asking what game(s) they want to go multiplatform. If that was the intent of the OP then I have to answer MGS4 since I enjoy the stealth genre and want to see what all the fuss is about, but don't want to shell out for a PS3.
