Atmospheric Thrust based on O2 level, and not altitude
It would add to the realism of the game if the ability to generate lift from atmospheric thrusters was based on the O2 level in the air, and not the altitude.
e.g. My friends and I built a mountain bunker, however our ship could not vertically take off, so we pressurized the hangar so the ship could take off, but we soon found out, despite having high O2 levels, the ship wasn't able to lift off the floor of the hangar as we thought it would be.
(We have not tested this in space yet, this is just something we just came across while messing around in survival)