The Obligatory WATER suggestion
Many folks in both the SE and ME communites have been yearning for liquid dihydrogenmoxide for years. Yes, I know it will proabably never get added to space engineers, but Medevial engineers is different; there is only one, fixed, planet for the game. And because of this there can be a bit more leway on how water can be simulated. Imagine powering a mill with a waterwheel and a dam. Or imagine players creating their own ships and sailing on large lakes; or fishing. The possibilites are endless. Please concider this idea.