I'd be surprised if ME3 is just about getting allies together for the big show down. I imagine most of that is already determined by the groundwork laid in the first two games. I suspect the core mission will be chasing after another Prothean mcguffin that'll either destroy the Reapers or drive them back for good. Remember that there was a mention in LotSB that the Shadow Broker believed that the Protheans were onto something other than the conduit So my prediction is that Liara finds something and sends you off on a new treasure hunt. I don't think the Reapers will be in any hurry to spread beyond Earth in a hurry, remember that it took them centuries to methodically wipe out the Protheans...of course back then they had control of the Citadel and the relay networks and could afford to pick the systems off one at a time... Which races show up for the big fight will probably only determine the state of the galaxy after the dust has settled; e.g. the Reapers are gone but...galactic civilisation is in ruins and it's going all Mad Max in space/the galaxy is OK but Earth is gone and humanity is all but extinct/the galaxy is OK, Earth will recover and the galactic civilization enters a golden age/the galaxy is OK, Earth will recover and all hail our new human overlords!