Elevators would be so useful for base construction. If you want a sky tower or a deep underground lair or something in the middle. having to climb stairs and ladders gets old and really limits player building and base design.
@BFT2020 true that! I usually take the stairs IRL. But in 7D2D, one doesn't get fit (or lose weight) by climbing stairs. And elevators would be a cool gimmick for base building.
IMHO, there should also be low tech alternatives. Lowest of the low would be rope ladders, that can be lowered and raised. A step up from that would be a manually operated platform, which can be raised/lowered by operating a winch. Actually, a winch could open all sorts of nice possibilities, such as a winch operated portcullis or draw bridge. And of course, the higher tier versions of the winch would be powered by a gasoline or electrical engine.
Naturally, this sort of thing would require quite a bit of work: new game mechanics/physics, new resources (rope, iron chain, steel cable), etc. But think of the possibilities!