Garrus: If you are a male Shepard, then he is basically your partner in crime and do-goodery. A sidekick, only with more of a presence and a higher kill count than most. He is a necessity, an integral part of the story almost as much as Shepard is.
Legion: Because of the story of the first game, and how the Reavers choose the Geth as their minions, having Legion return is almost a necessity as well. He is an in to connecting to the Reavers, and I imagine that there would be a very emotional encounter with Legion when they confront the Reavers.
Tali: The female sidekick and the only Quarian to be included as a playable character. Thus far, at least. Both of those lead to an almost sure thing of her returning. Especially because we have yet to see what a Quarian actually looks like without their masks. If the first one we see isn't Tali, I'll be very surprised.
Mordin: The fan-favorite, and for good reason. He is an incredible character, one that never ceases to amaze and amuse. The perfect mad scientist character for the science fiction era, and one that has become very integral to the plot. He helped perfect the genophage. To defeat the Reavers, the Krogan will probably have to be brought back. Thus, Mordin will probably be around to reverse the effects. I'd be very surprised if he didn't show up again.
The Rachni Queen: I know she wasn't one of the choices, but you know that she will turn up again. Permitting, of course, you let her live in your original Mass Effect character save file. Yes, I believe that shit will carry over. Imagine that, huh? Honestly, you'll probably have some sort of choice between bringing back the Rachni or the Krogans. That's how I see it playing out, anyway.
And, my guilty pleasure, Thane: Thane was an incredible character. He had the most emotional depth, I felt, and he never ceased to impress. You really cared about him, and he was always useful in combat situations. Sadly, due to the fact that he was already dying in Mass Effect 2, it's unlikely that he will return. But I'll be very sad if he doesn't.
The rest are not very important to the plot at all. Miranda may return as some sort of reminder of the days with Cerberus, but I can't really picture any other characters making a return without some big plot twists. Of course, knowing BioWare, they'll probably manage to squeeze every character from both games in there somehow. And, honestly, I wouldn't be adverse to that.
