If it ever becomes technically feasible to visually excavate multiple layers of a world, a lot of potential for new features lies in the ground. Firstly, to avoid the issue of players trying to dig infinitely deep into the ground and crashing their game, perhaps a maximum excavation depth could be enforced for settlements. It would start at 0 by default and increase by unlocking special techs such as the starting Deep Excavation or something along the lines of "Ultra Deep Excavation" or "Core Drilling". Different distributions of resources could be found or even restricted to different depths, such as Alien Artifacts becoming more common at deeper elevations but disappearing entirely after a certain level. The placement of certain structures could also be limited to specific depths, such as a more efficient water pump, an artifact extraction lab, or an oil rig (limited to specific world types). Buried alien/Abandoned World settlements could even start appearing rarely, featuring a randomized cluster of unique buildings still protected by extremely powerful automated turrets. Once a colony disabled the turrets with military units, it could start to occupy the settlement's buildings and take advantage of building-specific benefits such as massive education boosts or unique alien vehicle production. Buried settlements might also become hot points on multiplayer worlds, adding a new aspect to military campaigns. Though the engine mechanics necessary for this type of terrain manipulation would likely be impractical to work out for quite some time, I hope these ideas might serve as an inspiration for exploration-type features once the game matures farther.
Living the Spamlandian