OK so this may be just because I watched the movies lately but remember the enemy agent event? really annoying but a small scale danger event.
How about a Terminator event? A flash of lightning and a time bubble later and a naked pawn spawns. He is programmed to kill one of your colonists. Made out of bionic parts and extremely durable he will kill anyone else who gets in his way, take their clothing and weapon, and move on to his target. If you kill him you get some bionic parts and an AI core. Otherwise he kills his target and leaves the map to complete 'other objectives'.
I like it! I'm not sure about the dropping an AI core though. Seems a bit like an easy way to get a core if it's later in the game. If you get this event early in the game it could be brutal, but later on when you are prepared and equipped for huge raids it wouldn't be too hard.
Im thinking they can come in visiting groups. They wander inside your base and suddenly one starts killing. You cant interrupt its attacks, it tears through weak walls and doors, moves faster than any pawns. It wont ever destroy the colony but it will shake things up.
OK, that I like. It would certainly be interesting. It would give a good reason to keep an eye on those visitors that stop by.
With some debating it might fit into Rimworld lore, but is also very close to copyright. The idea of an assassin is something that has been talked about, and would be awesome to see. The idea of it still needs tweaking however.