Shepard could've died in ME2 and the PC for ME3 would be someone else. Just as there are many permutations of story elements up to ME3, there'll be many more up until the end of ME3 itself, including whether or not Shepard, or anyone on his crew lives or not.
I think it may even be possible, for example, to complete ME2 with almost all crew apart from Shepard dead. I hope the choices remain open right up to the trilogy's end. It is fun to replay with a different set of choices and a different Shepard. Doing missions in different sequence, different gender, different love interest, different class/skill specs, paragon/renegade with different choices made for Noveria, Feros, The Citadel, Virmire, Ilos in ME1 and in ME2 the various choices made to resolve various character quests and those on Illium, Tuchanka and Omega...there are hundreds, if not thousands of permutations which really make it interesting to replay and see what happens differently.