Bill Nye, Michio Kaku, Neil DeGrasse Tyson, and Morgan Freeman please!
Bill Nye or Michio Kaku would be awesome in small easter egg cameos. Neither are actors, so I'd be a bit riskier to cast either one in a major role.
Neil Degrasse Tyson would be cool as well, but he'd never agree to do it. He has a history of skewering Sci Fi films that are much more hard Sci Fi than Mass Effect for getting their science wrong. The only ones I've ever heard him praise were Contact and Interstellar, and the former was written by Carl Sagan and the latter produced by Kip Thorne. He'd probably sooner trash Mass Effect for being schlock than voice anything in it.
Michio Kaku would probably be easier to land. He did participate in the Science of Mass Effect thing they put out a few years back, and mostly kept a straight face while doing it.