My first thought is there's a couple of guest stars from Season 2 that I'd love to see make another appearance. First is Jaimie Alexander, who played Reva Kline in a couple of eps. Of course her movie career is taking off now, so getting her back might be tricky(and expensive), but she was a fun character(and I'm a big fan of the actress who played her). The other is Tim Guinee, who played the instructor at the farm in the episode where Annie went back to find out who was burning trainees. He gave that character some nice depth in a short period of time, and he had good chemistry with Piper Perabo(not romantic chemistry, but he made a good mentor/father figure type). It'd be fun to watch his character and Annie work a mission together.