Ok, just saw the trailer. Just snippets still satisfied.
Also, since we are talking about the Ark theory, here's what I came up with a while back.
Since we are talking about this stuff, here is what I proposed a while back:
''Just wanted to push this idea a little bit further. We know that in the Milky Way, there are many parts that are unexplored or inaccessible. The Council banned any new exploration after the Rachni incident. Who's to say that other species aren't living in those areas?
Just look at humanity. Because the Charon relay was frozen, nobody knew about them. If humans hadn't turned on that relay, nobody would've discovered them.
Also, don't forget that most species have sent expeditions into deep space without ever hearing from them again. In case any of you remember this story: Alliance Links Lost Colony with Manswell Expedition of 2070
A cool theory would be that just before the final battle with the Reapers, the Council species secretly sent out a sort of Noah's Ark with all the species in a last ditch effort to preserve their future. They happen to land in an area inaccessible with the Mass Relay's, which saves us from having to deal with the ending choices. They then have to try and remake contact with the rest of the galaxy while dealing with new environements and species.
Just throwing out some ideas''
Adding to this: Liara says we aren't very widespread, and very little of the galaxy is explored. To make it more ''Mass-Effecty'' You could even start the story 100-200 years down the line, and our species (Asari, Turian, Salarian, etc.) having established colonies in that unexplored region. Add some inter-species tension(cold war), some new environements, and a few new (and potentially hostile) species, and BAM.
You get a great game AND something that will not set a nuke off in BSN;)

Hmmmmm...
A Prothean?