I'd like to see Adrian Shepard make an appearnce in Episode 3. Since we've seen that the G-Man is more or less losing control of Gordon and the situation (as described by Valve) Adrian would make for a good tool for him. The G-Man left him stranded on a train car in that sub space region between worlds.
Definitely. I've never played Opposing force past a couple stages, but from the ending, he' a prime candidate to be brought back into the mix. And it'd be extremely interesting considering he'll be a character (other than Chel, who's supposedly going to be in Episode 3 as well) that unlike Barney, is a character who the player has controlled, but has never seen from the third person.