you kind of alluded to what you wanted to do, I'm not sure how you are going to do MP. I would Love to see an Open 'world' on a Galaxy scale where you pick a star system and terrestrial body to orbit around to build you space station(of course you can move it to a new location at a cost or so). Where you pick is important, you build along a trade route you get more transport, escort, pirating/privateering missions than the other mission types.
The building system is neat as it is, but a common issue I run into with SE (space Engineers) is how restricting it is. It's fun to place blocks and mess around but now 1000 hours in I'm looking at several hundred ship builds that pretty much all do the same thing. ya one Might be fighter and the other a transport or a repair ship, but they all have the same power output to reactor ratio, the same thrust to thrust ratio. Nothing is unique, my repair ship welds slower than his but moves faster. woot, would it not be cool if not only does it move faster but it boosts the resistance to damage types due to the welding style? it may take longer to weld but the ship stronger than stock models. Or ( and I think you talked about this in one of your galactic line vids) the targeting computer in my turret has a reduced fire rate but high accuracy.
You may be able to avoid the "its too complicated" issue by giving a base module that gives just straight stats. (it just gives power x1, where as someone who has played for a bit needs a specific build for a ship, so gives power x3 with a 30% increase radiation output) very RPG like but RPGs are fun
another trick would be the ability to adjust the effects even further (3x power 30% radiation increase, boost the power higher radiation lower the power to 1x 0% radiation)