Now this is a stretch, but lets take an honest gander.
Liquids and gases, the voxel system in space engineers has always been a great addition since the first asteroids, an absolute joy. But I think its time we talked about more than just rocks and dust. Id propose adding into the games poor ol' physics engine, some liquids and gases, things like water, hydrogen, oxygen, maybe even other resources already present in the game. It just felt right to think, well earthlike planet should have earthlike circumstances, this could lead to new dangerous things like CO2 death, reactor fumes and spreading fire, resource storage, new ways to fly, new ways to power our vessels with hydrogen like actual fusion reactors not reliant on uranium. Said materials would obviously need a way to be mined, like air or water pumps, maybe scoops for space particulates and the like, obviously this is all a huge long way off and would definitely take some serious work to get the physics engine to handle such a thing, but hey this is all about random fun ideas. Go for a swim? Underwater base? Floating gas station on jupiter? Would love to see anyone elses ideas on this. ^^^