Aside from building spacecraft and civilizations, what if we could build machines? Machines such as turrets, humanoids, mech robot's we can operate, drones etc. I know I'm getting ahead of myself but it would make for an incredible DLC.
It may stray from any ideas you have already but I think it would be interesting if a bounty is placed on players who commit crimes. Every player that falls under the category of criminal will be placed on a list accessible to all and hunted for the posted amount.