I know many who love Agent Simmons. I for one liked him in the first TF but thought he was a tad ott for ROTF. If he returns hopefully we won't see him breakaway his pants.
I really liked Miles (remember him? Sams bff in TF1) and would love to see him back even if it's a cameo. I don't think Sam should forget his good buddy.
Rumor has it that Figgs is supposed to return. The only reason he wasn't in ROTF was because of schedule conflict. Hopefully he makes a return in TF3.
I'm sure that Epps and Lennox will return. Would also be cool if Jon Voight returns as Sec. Heller. Maybe because I am a huge Voight fan.