I was bored, and started thinking about who they should cast. Then I happened to be watching Eureka...
Erica Cerra would be perfect for the part of Ashley.Seth Green obviously for Joker.
Wrex and Garrus will obviously be CGI, so they should just keep the VO's from the game. Not sure whether Tali can be portrayed with a costume though, unless they use CGI from the waist down and digitally remove two of the actress's fingers.
I really could care less who plays shepperd, though since bioware consistently portrays him as male, I'm guessing that will carry over to the film.
No, they'll probably make a whole new story set in the ME universe, like the books and comics. That way, they won't have (as many) whiners, who demand femShep with a relationship with Leigon. Of course, they make take some parts of the story where one got to choose events as 'canon', as in you saved the Rachni, Wrex got saved and is lord of the Krogan, the paragon eneding of ME2, etc. Essentially the 'good' ending, just as, in Star Wars canon, it was the light side that was the 'correct' path in KotoR.
Except they never do that, because film industries could care less what fans think, as long as they buy tickets. The reason they want to make a movie is because they saw the game and said "wow. I could totally make that a movie without having to figure out an idea for a plot on my own." It's guaranteed to have interest, since the games are such a success, whereas a new movie with a brand new idea is financially risky.
They'll go with what's familiar, because its easier, and because they figure that most fans of the game want to see the game on the big screen, not something that merely refers to a couple things that happened in the game. If it didn't feature the Normandy and its crew led by Shepperd fighting to stop Saren and Sovereign, what would be the point? It wouldn't be Mass Effect. As to what choices are made in the movie, that will be up to Legendary Pictures and Bioware.