^The trouble with that is that by all accounts, there's going to be a very wide variety of possible outcomes for ME3, making any sequel when you can import your old saves somewhat untenable. They'd be forced to choose a single 'canon' storyline and move from there, which would be a shame considering how personal the Shepard trilogy can be to the player.
Which is the same problem many games with multiple endings have like KOTOR 1 and 2, although KOTOR 2 made it a choice as to what happened in the first game. What can be done is have the player go through different points of view depending on the choices made, all of them converging into the climax as seen from the different points of view. Then you have your storyline intact and it's only a question of how the player saw it happen.