I've very recently spent a close to a hundred hours on a single character on Caves of Qud, and I've hardly scratched the surface of what would be it's endgame areas. If you can handle the interface it's an incredible game with near endless possibilities. The build variety is pretty massive with the way mutations/cybernetics and skills/stats interact with each other, allowing you to build your characters exactly how you want.

Tales of Maj'Eyal is also an excellent choice with a huge variety of ways to play with it's dozens of classes and a wide variety of races to play to keep things fresh each run. The world itself is static, but each zone itself is random per run, and it also has the ability to tune the difficulty a bit to your liking, as well as having a couple expansions to check out too (with another on the way).

Cogmind is another one I've put a decent amount of time into that has a ton of depth, allowing you to pick and choose what parts to strap onto your bot for each run. Stealth hacker runs, ranged artillery explosion builds, incredibly fast ninja melee builds, there's a myriad of cool ways to experience all it has to offer.