Oh man oh man the thought of that is beyond exciting I don't care what anyone says his voice acting in Oblivion was great and with a greater level of voice acting experience it could be epic. He'd be great as an Admiral or as the father as the player or on a lower role it would be fun to talk to him as the navigator, XO or whatever of the ship.
I can kinda imagine Chris Hemsworth in the role of a soldier or combat based companion but that's not really an epic level like Patrick Stewart. Leonard Nimoy would've been great I still love his CIV 4 voice acting it just made the game for me but obviously his passing. So I guess my suggestion would be Jeff Bridges everone might know him as "The Dude" but his roles as father figures and as a wise man in recent films would perfectly translate to voice acting in Mass Effect maybe as an auxilarly crew member or what I'd find more appropriate as an Admiral, another ships CO, some form of a military leader.
Edit: Actually I wouldn't mind the inclusion of Benedict Cumberbatch he's done voice acting and he really gets into it as if he was acting in front of a camera, plus he's got an epic voice and is a great actor who could fufill so many roles.