I say old chap said:
I'd really like a rainbow campaign game where I could be the terrorists. Plot changes partially based on how well your terrorist group does. Do you get blitzed, hold out and die well, win? Campaign moves forward from there.
Make the rainbows damn good, but possible to trap and outwit. Jump perspective between all your terrorist mooks, with some being awful shots and quite anxious to others being cold killers. An evolution of the tower defence game, in third person?
Yeah, would be nice to play defence for once.
Mind you, I'd just always stick everyone watching a few bottlenecks from various different angles, and/or stick heaps of terrorists in with the hostages, because you could easily make things borderline impossible for Rainbow.