[Tribble-eater]

How about a new species that syphons off the life force of other species and they can do it on a planet wide scale "Not Earth" its been done to death
Oh wait that that may be to close the Stargate Atlantis
Maybe some aliens that are like current terroist cells attacking individual planets and are hidden and have to tracked down to get to the central leaders
that would give the op for many battles and Would love to see how Kirk would handle the situation

Or Just let Simon Pegg Loose on the universe where everyone would rather slit their wrists instead of having to listen to him