Yeah, guessing here that main plotline will begin on Milky Way at the middle of the Reaper war, some high military guy (Hackett?) gets your ass into building an Noah's arc with some kind of new found FTL technology amidst all the confusion around you.
Very high secretive ****, main guy (the N7 guy) gets the job done of getting most genetic pool and you're one of the officers aboard this project.
There'll be Voyager-type of shenanigans between surviving Krogan and Salarians / Turians, because doh. I'm guessing the Krogans got on board either by force or by hard-line negotiations (you can use our resources for this if we're into it as well). Lots of potential here for you either to appease these different tribes or even for some of them going rogue on you, etc.
At some point, N7 John Wayne guy goes rogue too, and now not only you have to fight that bastard, you'll have to outrun him on your chase to do [MYSTICAL ARCHAEOLOGICAL FINDING] before something [DEEPLY DISTURBING] happens.
Yeah, it will be Saren all over ****ing again. Somewhat. I'm speculating.
Any guesses on who's gonna cameo? Shepard him/herself? Hackket? Mordin the science guy? Because, come on, there're gonna be cameos!