True. I like pretty much all of the ones you listed and think they would present their own challenges.

The ancient danger, 20 healthy prisoners, and pirate the pirates all sound like a hard one that would be fun. One I was thinking of was a Die category where the goal is to die as fast as possible with the extra rules being 1.You can not banish any colonists or indirectly banish them by forming a caravan and abandoning a colony with colonists inside. 2. (maybe this one) Your colonists can not kill each other. 3. The time ends when the game over screen is seen. Though, it might be a bit too easy to die.