Good thought exercise.
Gamecube and NDS original and lite have some interesting GBA options, and Wii has VC and Gamecube backwards compatibility.
But I have to go with:
Starcraft 2: Has Broodwar mechanics as UMS maps. No multiplayer hurts, but high skill ceiling and engrossing nature means it shouldn’t get too boring. Include Alphazero AI computer opponents with scalable difficulty, for realistic opponents.
Hollow Knight: With mods, randomizers particularly. Scratches platformer and Metroid style itch. Speedrunable.
Grand Theft Auto: San Andreas: Tonnes to do, and ways to approach this game. I have yet to finish a master save file (everything you could possibly do before the first mission), but that’s been fun as hell in itself. Has good car race mechanics. Has good visual mods too.
Mine-fricken-craft: Redstone contraptions, and artistic outlet could be cool, but perhaps my cell has real-world artistic expression opportunities?
Procedurally generated worlds somewhat prelude stale play.
Morrowind (or some other significant RPG) for multiples different class builds, challenge runs, etc.