The way I see it a one-person TC can be done in one of two ways (or a mix between the two):
Theme Park (a lot of name changes, a few stat changes, a few worldgen changes, but mostly the same stuff).
Mini-TC (an actual total conversion, but with limited scope to make it buildable: essentially a whole new game on the same core mechanics).
So, with that in mind, here’s a few ideas by category:
-DOOM II: Cataclysm on Earth. The plot and monsters of Doom 2, now with 100% more Cataclysm. You’d need to build a hell-level generator, because It’s DOOM, You Go To Hell.
-Star Trek: Cataclysm. (Not necessarily Star Trek, but the classic space opera ethos.) You are stranded on Planet Cataclysm with nothing but your communicator and your Space Fleet regulation minidress and gogo boots. Your objective: don’t get eaten by the local wildlife. You might even possibly include a victory condition where you assemble and power the appropriate technobabble to contact Star Command for rescue, if you can live long enough and raid enough dungeons for random parts and artifacts.
-Robot Apocalypse. The machines have risen up and destroyed all humans. It’s up to you to destroy them right back! (Or forage for food and water so as to not die.)
-Apocalypse Robot. Humanity has been destroyed by a tragic Zombie Apocalypse or something. You are a robot built in the last years of the now-dead civilization. Scavenge for fuel and spare parts, fight off mutants, other robots, etc. Shut down out-of-control killbot factories (repurposed triffid groves, etc). Upgrade yourself to become the super-robot you’ve always wanted to be.
The Base: cataclysm has struck an isolated outpost in a hostile environment (outer space / under the ocean / the antarctic / etc), which is now falling apart and letting the hostile environment in. Survival means a continued battle against the ever-encroaching environment. Plus there are monsters and stuff.
The Lab: Science Gone Awry has left you trapped in a hellish underground laboratory with only your wits and SCIENCE to protect you. Classic survival horror dungeon, ramping spawns -> evolving Science Gone Awry monsters.