Make a tower defence mod.
Similar to Colony Survival, there's already a mod to place archers and the such around for defence, what about a mod that once you gain a house you begin getting attacked by weak forces and the more you build the more enemies come. Once you've built x amount of war structures for example, you could have the enemy bring in some catapults for the player to find a way to defend against as well as the barbarians.
In a game like this as well as space engineers, what i'd really like is AI engineers to befriend or declare war on. AI's that have the same or similar building and defence abilities as the player does, that expand to new areas and pay taxes and you can raid their lands and the such. It would be good for multiplayer too, then there would be a reason to play and gang up on the computer AI with some friends lol.