well my idea is this"there is a heavily armed religous cult that has taken hostages, as swat HRT w/ever floats your boat you must Save the hostages, disarm some explosives, 'destroy' the weapons cache(spare guns with smoke grenade) and eliminate the cult leader, as cultists you must prevent all of the above and survive till eplosives detonate" I am thinking best terrain is a multistory building with lots of outdoor cover
This actually makes me think of a mission in COD4, where you and a few others have to clear out a small base pretty much (destroyed cars, three or four buildings, walls the whole bit). Maybe it would be possible to integrate something like the layout of the area of the mission into that.