Ok lets see hmm...
John Shepard must be portrayed by an charismatic, influential, overall actor that is able to completely copy personality of Shepard.Shepard is complicated character.Actor must copy a man favored by thousands of systems and in same time man able to influence most dangerous, smartest, powerfull individuals in ME universe.That requires great charisma, acting skills and experience.
Honestly I'm not sure what actor will be able to do that.Sam Worthington maybe, but he needs to work on his charisma alot.Christian Bale perhaps, he is maybe perfect choice.As for the others well:
Miranda Lawson -Yvonne Strahovski ofc > She is Miranda and I cant think of any other actress to be Miranda.If she is not Miranda in movie i wont watch it.
Joker - Sure Seth Green was good at giving his voice, but his looks are totaly oposite to Jokers and I dont think he would be able to take roll as best pilot in Milcky Way.I don't know who should play him tbh.
Jack (Subject Zero) - A very interesting character and I would love to see Nataly Portman in that role since she never played someone as badass as Jack + we knows she doesnt mind having a hair cut
Jacob - Sure Adam Lazarre-White was good in that roll, but he can easily be replaced and wont miss much.Not sure by who, maybe Denzel Washington.
Illusive Man - Its Martin Sheen ofc, end of story.
Kaidan Alenko - His role can easily be filled with some unknown or less known actor, for example.i think Mark Ruffalo would be quite okay in it.
Ashley Williams - Ok I'm really not sure about this one.I would have Holy Marie Combs in consideration, but not sure at all.
David Anderson - Keith David is ok, but can also be Samule L. Jackson
Ok now alien characters.There would be some like Tali, Wrex/Grunt, Garrus, Mordin that would require just voice acting and their actors are quite good.As for Asari, Drell.. that can be played by make-uped actors well :
Liara T'Soni - I would give that role to Anne Hathaway.She is amazing actress and her physical appearance would fit perfectly.
Thane Krios - Ok, when I first heared krios voice acting I thought "What the heck, is this Liam Neeson?".I was suprised to find that it wasn't, and I think that role should go to him.
Samara - Sigourney Weaver would fit nicely, maybe Gillian Anderson at long shot.
Ok that are ppl i would consider for upcomming ME movie.We all know that most of game adapted movies resulted to be less than junk, but I have my faith in this one, or atleast hope.Tbh Bioware crew should write scenario for this one cause they are very good in it.Their Star Wars kotOR series owns Lucas' movies by far in storyline, twists etc.Go Bioware!