If only we had a Generic Universal RolePlaying System. You could even give it a clever name, like G.U.R.P.S.
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Or HERO System if you want to be heroic (and really crunch those systems)
I bet you could build something off of Starfinder.
Yes. Starfinder has planets, and some planets have caves.
Yes! Exactly! And some of the aliens in those caves might be somewhat monstrous!
You could maybe just remove the mechs from lancer. Maybe like double the health pools of pilots so they fight longer
Getting stronger while you play? Like with weighted dice?
I hear Dungeon World is pretty good!
It would be a stretch but I think Dawn Patrol could work.
Theres a cool game called Slugblaster which has a similar concept, except that you're going into alternate dimensions. I am sure you could handwave the dimensional travel as caveing
Paranoia is always underground, you just need monsters.