We've already had 'famous stars' in Mass Effect (TIM, Anderson, Hackett, Aria etc.) What did you think about them?
The only person I can think of who fits the 'big star' mould in a Mass Effect game is Martin Sheen as TIM. He was great and I enjoyed his performance, but I wouldn't like someone so famous voicing a much heard character like Shepard or Garrus as it could be immersion breaking.
As for the others they're regular actors who've been on tv and in movies, but aren't A-listers. I still think that anyone who is too big of a star would distract from the role they were playing. So if you did have, I don't know....someone from Harry Potter or Morgan Freeman or Liam Neeson like ppl have been saying, I'd find it a bit weird. Plus seriously, they don't need the work, give someone who is passionate about voice acting the role.
I knew Captain Anderson from tv shows, but not enough to even know his name irl. He's one of my favourite vocal performances of the entire trilogy. Love Hackett too, but not someone I consider super famous. I saw him in an episode of Castle not long ago, I kept willing him to say Shepard....damn his voice is epic.
What about faces? I for one hope they don't do the face/voice thing they did with Miranda again. I watched Chuck and she starred in that (Yvonne Strahovski) and it was definitely immersion breaking for a while. Although now I'm just like 'oh hey Miranda' but still, motion captured faces rarely move in realistic ways, they always look odd to me and take away some of the artistic possibilities from the developers.