In the "Making Of" DVD that came in the ME2 CE, they talked about it being the "Empire Strikes Back" of the trilogy. That would make ME3 the "ROTJ" of the three. If I'm not mistaken, in ROTJ, the Rebels took the fight to the Empire. I would have thought a better plot would be "Human-E.T. alliance travels to whatever extra-galactic, darkspace dimension that the Reapers live in to wipe them out once and for all" . Instead they decided to go with "One Reaper. One Reaper. Oh shit! It's all the Reapers!"
Thats assuming that the invasion of earth is the core of the plot rather than, for example, the catalyst. The plot could very easily go with the fall of earth as a teaser then the rest of the game is making strategic strikes building up to taking down the reaper king in darkspace or something