b) achievement system (for long time fun / time killer / task variety). Kill 100 livestock, swim 20.000 blocks, get every ressource of a planet, etc… with respective rewards (decoration, receipts, titles / names, etc.)
Here you can very easily add a lot of stuff, that motivates players to follow other goals for some time, even when they miss new “real content”. I also implemented it under 3. since for me currently there is no real system yet. I think there somewhen was a very nice thread in which many many ideas were collected.