1. Garrus. He's the action-hero, it's very natural to think of the sort of things he's doing and he's Turian.
2. Tali, she'd be a nice change up of protagonists and she Quarian which would be interesting. (Also quite cool to have a techy)
3. Legion - I don't know how they would make it work, but Legion rocked and it'd be interesting to get a geth perspective.
4. Mordin - It's hard to see Mordin be quite so actiony, but he's got an interesting mindset and is Solarian.
5. Zaeed or Wrex - They're both a little gruff and maybe a slightly too dumb or placed badly in the universe to see it working out as an adventurer (Wrex was all about long term goals by the end), but they're fun people.
Bad choices
1. Miranda, Jacob, James, Kaiden. They're human which is boring and their personalities aren't very dynamic either. Ashley is a little less boring, but also human.
2. Liara. Bland personality, and the Asari are the one race I don't particularly care to learn more about. Darn space elves.
3. Samara, a little too mature and a little too crazy to work out. Also an Asari.
4. Thane. Thane technically could be good. The drell are interesting and his line of work is interesting (although it would mandate ME4 as a prequel) but I think he's a little too nice for it to work out.
But I can't really see it happening, there's too much potential continuity snarl and Bioware games have always been power fantasies about a person you created, I don't think they could pull off a MGS2 and they couldn't pull off a game with a defined protagonist without losing a lot