A game mode that allows you to keep playing after death. There are two ways this could go:

• Continue playing as the monster who killed you. This game mode would likely be non-scoring, and your actual character’s death would be written out in the normal manner.
• Continue playing as your own corpse. In this mode, you are effectively a blind, deaf, and sessile monster who can take no actions other than resting. The game finally ends when your corpse rots away. This is mainly interesting combined with an implementation where allies can zap your corpse with undead turning to resurrect you; if they do so before you rot away, then you can keep playing. It’s not clear what would happen if you are petrified instead; a statue can survive indefinitely, and the odds of a pet being able to stone-to-flesh your statue are low but not zero, so possibly the criteria should instead be “you die when there are no more pets on the level”. This still has a problem in that some pets could survive indefinitely, though.

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 new game mode in which everything goes wrong for everyone. Some examples: wands explode 50% of the time when used, all thrown/fired weapons can fly off randomly as if cursed, critical misses happen a lot, 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”.