Convert the sleeping gas trap to just “gas trap”, and make it scale based on the difficulty. Each individual trap releases a single type of gas; which type is partially dependent on the level difficulty. At low levels, they all emit sleeping gas like normal. At higher levels, they emit stunning hallucination gas like container traps, clouds of poison gas, and maybe even nastier gases that reduce speed or miasmas that cause illness.

Allow for gas clouds of different types (confusion gas, sleep gas, hallucination gas, etc) by passing an additional argument to create_gas_cloud.

Clouds of sleep gas would immediately have a use in Nazgul and orange dragon breath attacks, which are already described as such. They might also be useful for sleeping gas traps, which would emit an actual cloud of sleep gas. Depending on implementation, the sleep gas cloud could have only a chance of inducing sleep rather than always knocking you out, which could make it tactically interesting to navigate/teleport/etc rather than just getting put to sleep and having the cloud gone by the time you wake up.

Steam vortexes leave behind a trail of non-poisonous, temporary clouds behind them as they move. When they are killed, they burst into a large cloud centered on the spot where they died.

When a monster and a gas cloud are on the same space, and you are adjacent to that space and non-blind, the game should render the monster rather than the cloud.

Fog horn, a magical tool that when played surrounds the user with clouds and possibly fog clouds (the monster).

It would be best if some type of non-poison-gas cloud were implemented first; it is probably a bad idea to actually change the terrain around the user into cloud. It would also be good if the obscuring cloud were useful at blocking monsters’ pathing to the player; currently they would ignore it so creating a fog cloud would mostly hinder the player.

Assuming the game supports creating temporary regions of non-poisonous gas clouds: New “smoke bomb” potion. It makes you choke/vomit when quaffed. Intended to be thrown; where it shatters, most of the surrounding terrain will be transformed into cloud.

Vanilla has since added non-poisonous gas clouds (for things such as mist from zapping fire over water), and this effect could work just as well by breaking a smoky potion, instead of adding a new object.