My maximum i think was floor 4 or 5 so far, and when i restart i feel i'm doing the same too much.
It's not about difficulty really, it's more about doing the same. Progression is too slow (xp, skill unlock...) to feel i can really do "naked but new".
I've always find "achievements" (as Steam's ones), completly useless for me. I absolutly don't care about achieving something on a list.
I do like ingame perma-progress, like maybe run 1 is random, run 2 you can start with one of the previous weapon, run 3 you can pick one of the run 1 and 2 tools, etc.
And skills. Maybe start with 1 skill line already up, from the previous skills you've already played with and raised ?
What about a level unlocker ? Each time you reach level 5, 10, 15, 20... you get to save your toon to start with it if you die. So if i reach level 9, i can start again from level 5 with the toon i had at level 5, and i only have to redo level 5+ until i reach level 10, etc.
Or not, it's your game, and it's only my opinion and my taste.