Matthew Fox as Commander Shepard- Not necessarily an original choice, but I still feel that he is the right one. He looks like the traditional Shepard, he proved he can act, and he won't have that high of a price tag, which is always appealing to studios who embark on these big-scale projects.
Olivia Wilde as Ashley Williams- I struggled with this part for awhile, but then I netflixed Tron: Legacy, and felt that she is the right choice. I don't know if anyone can play Ashley perfectly, since her character is so complex, but Olivia should still do pretty well.
Doug Jones as Garrus Vakarian- He was a perfect Abe Sapien, and Abe proved he can act in prosthetics, which would be my choice to how Garrus should be done.
Cillian Murphy as Kaidan Alenko- OK, he looks and sounds nothing like him. But Kaidan isn't that complex or necessary a character, and I thought that if a talented actor could play him, he might bring more depth and heart, and make us actually feel sorry when he dies on Virmirre (since he really should instead of Ashley)
Gemma Arterton as Liara T’Soni- She is a talented young actress who can provide the right qualities to the character. I also thought of Liv Tyler, but I thought that a younger and cheaper actress is more feasible.
Amanda Seyfried as Tali’Zorah nar Rayya- She is in pretty much everything these days, and she seems to be able to perform this type of character. Not that this role is hard anyways.
Michael Clarke Duncan as Urdnot Wrex- He was definitely one of the better parts of Green Lantern with his performance of Kilowog, and if he brings that and dash in a little Lt. Worf-ish qualities, and he's got this role in the bag.
Seth Green as Jeff "Joker" Moreau- Some roles don't need recasting, and really, who else would you want to play the role?
Keith David as Captain Anderson- Again, if it ain't broke, don't fix it.
Ian McDiarmid as Donnel Udina- Always nice to have a sci-fi veteran in these type of movies, and it's not like he's busy right now.
Michelle Pfeiffer as Matriarch Benezia- Watch her in Stardust and you'll see what I mean.
Jeremy Irons as Saren- Great villainous voice for a great villainous role.
And, in terms of sovereign, I think the sound effects guys should come up with his voice. No voice actor is really necessary.
Well, there you go. Let me know what you think in the comments below.