Add a “randomizer mode”, a non-scoring mode enabled by some option you put in the config file that randomly mutates the stats of each monster – for instance, in one particular game, gnomes could have fewer hit dice and thus lower HP, while soldier ants gain an ice attack.

In the spirit of traditional randomizers, this could also randomize loot, for things like artifacts and invocation items being mixed up and findable in regular chests, monsters generating with and dropping gear they shouldn’t have, etc.

Add a game mode that spawns you in Sokoban on the stairs to the main dungeon. No monsters or items generate in this mode, it is just for practicing the solutions to the puzzles.

Streak mode that prevents you from having more than 1 save file at a time.

Challenge mode in which all non-sessile monsters are faster.

Reflavoring of Heaven or Hell mode: everything explodes when it gets hit by anything. A.k.a. “HackSplosion”.