I'll give anyone who can think up a viable way to cover this plot hole a cookie.
Mass Effect 3 is so doomed narrative-wise that we can pretty much go ahead and damn it by the image above.
Why is it damned? Well it's simple.
The grand enemies of the series, the Reapers, are morons. If their numbers "will darken the sky of every world" (http://www.youtube.com/watch?v=JvrIFIjTGt0&feature=player_detailpage#t=215s) then why in the hell are they ONLY attacking Earth? Obviously that's just speculation, but it sure looks that way. As soon as they reach a Mass Relay (there's one in our solar system), they propagate throughout the Galaxy essentially buttfucking us all.
SO, as soon as the Reapers hit the solar system, they'll employ FTL (Faster than Light Travel) to make it to the Charon Relay, which is approximately where Pluto is. We know they're at Earth and Pluto is about 5.7 bill km from Earth. Let's say Faster than Light travel is 300,000 km per second, which is like 200 km more per second than lightspeed. At that speed, it'd take them about 5 hours to make it there and kill everyone in the Galaxy.
So in the end, they're gonna have to come up for a super fancy explanation as to why the thousands upon thousands of Reapers are just chilling on Earth. I mean one of them about destroyed the strongest group of ships in said galaxy, so thousands of them will carve up Earth in a matter of days.
Also further supporting my Reapers are retarded theory is the image where you can see they're carving up Africa. There's nothing of value there!
SHIT REAPERS GET IT TOGETHER
I'll give anyone who can think up a viable way to cover this plot hole a cookie.
Bioware and EA are playing on the general publics affinity for planet Earth and only advertising/showing the Reapers and their attack there. With the general public's mindset being "Holy shit their going to kill the humans.." many fans will feel a need witness the possible destruction of their character's native home world. Upon Mass Effect 3's release I'm sure throughout the game you'll hear about/see all kinds of planets being razed. More than likely you will probably be able to damn or save several planets/colonies/civilizations with the choices you make.
COOOOOKIE?
yeah sure
I guess I should've prefaced my observation by mentioning that they're obviously gonna pull something out of their ass. I just hope they don't half-ass it too much and at least try to use something that they've already mentioned in the previous games. ME2 was a fantastic game, but did a lot of Riverdancing on the canon for no reason other than laziness. AH WELL, I'll have fun with ME3 either way.
I support mine and Andrew's ending to the game:
Captain Anderson rips his shirt off Hulk Hogan style and simultaneously engages every Reaper in hand to hand combat while "Real American" plays in the background.
AND WINS
GAME OF THE MILLENNIUM!
My theory is that all of the Reapers die of old age just as they are approaching Earth. The humans tell the council that they beat them and they will not reveal how they did it. The humans then threaten the Council with their "secret weapon" and now the humans run the Citadel.
The end.