If you enter a room containing open pits of lava, print a message “It is hot in here.” If hallucinating, follow this up with “…but it’s a dry heat!”

On the Plane of Fire, run a rudimentary cellular automaton to make the lava slosh around, with a couple rules like “won’t encroach on radius 1 or 2 from the portal” and “there should be no net gain or loss of lava across the whole level”. It being static and the same every time except for the portal location is rather boring.

Shorten Gehennom drastically, paring it down to a few mandatory levels (the Valley, the Wizard’s Tower (containing Vlad’s Tower entrance), the vibrating square level containing the fake Wizard’s Tower to portal into the real one) with no demon lord lairs and no filler. In exchange, once you start escaping the Sanctum with the Amulet, any level you reenter for the first time gets partly overwritten with Gehennom elements – rivers of lava, twisted mazes, and in particular, the demon lords. However, enough areas of the map will be left in their original states so that it remains recognizable. In particular this is aimed at making sure any stash you have made may not survive the process.

There are several motivations for this:

• Reduce the common feeling of “the game is over after the Castle” by putting only a few things between the Castle and the ascension run.
• Put the main challenge of Gehennom on the way out of the dungeon, not on the way in.
• Make it uncertain that the player will be able to go back to a big stash of items after grabbing the Amulet and take whatever they need before entering the Planes.
• Reduce the number of situations where the player can simply levelport away from a hairy problem such as a tough demon lord, collect the appropriate gear to fight them, and return (dipping into and out of Gehennom).
• Remove a Gehennom-worth of items the player finds on the ground and in monster inventories (where “the difficulty is flat but the loot keeps coming”).
• Mitigate the tedium of knowing you’re as powerful as you’re ever going to get but still have to retrace 50 explored levels. Relatedly, remove the incentive – the ability, even – to go tunneling through mazes.

The game should not assume heroes all carry an unlimited invisible box of matches to light candles and lamps with. Doing that should require a fire source, whether that is Fire Brand, lava, a fire trap, the wand of fire, the fireball spell, or an already lit candle, lamp, or potion of oil. There could also be a new “lighter” tool which enables you to light things, but can get lost or run out of charges.

Make the Plane of Fire more interesting by:

• Getting rid of the fire traps (which mostly serve to provide lots of message spam).
• Making the entire plane constantly deal damage if you aren’t extrinsically immune to fire, more if you lack both extrinsic and intrinsic resistance. Being close to or over lava hurts you even more, and can deal damage even if you are immune or “like fire”, possibly based on how many of the 9 squares immediately are you are lava.
• Lava can erupt randomly from lava, which does major damage for levitating over it.
• Ensure the portal cannot spawn right next to the arrival location.

A cursed scroll of fire should create a pool of lava underneath your feet. Possibly only in certain circumstances like with negative Luck.

Force-fighting a monsterless pool of water or moat or perhaps even lava while wielding Frost Brand should freeze that square into ice (or ground if it was lava).

If Frost Brand falls onto lava, it instafreezes the lava rather than sinking into it.

Lava gremlins: a different gremlin species that reproduces in lava instead of water. It has at least one physical damage attack and at least one fire attack, and the regular gremlin intrinsic-stealing attack, except they steal intrinsics during any time that is not night.

Lava on the Plane of Fire regularly erupts in a spray of molten rock, which is extremely harmful and thus very bad for you if you are over it or adjacent to it when it happens. This makes it less appealing to just levitate or fly and completely ignore it.

Fire and frost horns, rather than shooting a bouncable ray of fire or cold, cause a powerful area fire or frost effect centered on you. The intensity diminishes the farther away from you it gets, but it hits every single creature in its fairly large radius (except you).

This effect could also freeze water and lava or evaporate pools in the radius of effect as well.

Some object that you can dip in lava to enhance its properties.

Option to invert the colors of liquids, so the dominant color of the character glyph is the liquid color.

Sokoban levels with water or lava instead of holes (and make it so that boulders never sink without a trace in Sokoban). Air currents will still pull you down into the liquid if you try to go over them. Not specified what should happen if the player tries to boil away or freeze the liquids.

After the Valley, have a guaranteed special level “The Gates of Hell”. This level design is very clear: “This is hell. These are the gates, and they are closed. You are not wanted here. Go away or perish.” The level is a large cavern, with a massive arc-like fortification across either the left or right side. The gates are implemented with both iron doors and a drawbridge across lava. There is quite a lot of lava, and a lot of demons inside and outside the fortification.

Gehennom level that is cavernous but with 1-3 defensive fortifications consisting of a slightly twisty undiggable wall with turrets or guardhouses staffed with demons, and lava moats with drawbridges. Stairs on opposite sides of the fortifications.

If there is a web over water or lava and you are a web-walking monster, you can move onto that space without falling into the liquid. Possibly if you’re any monster, not just a web walker.

Any lava plain levels that generate should grow the region of normal floor by 1 in each direction, so that the player does not have to walk on narrow and diagonal paths through it. Ideally, this should be provided as a flag to any cavern-fill levels.

Object property “frost walker”, which only generates on boots. When worn, lava cools into solid floor and water freezes into ice in a radius 1 around the wearer. Optional if they unfreeze afterwards. Alternatively, make this an artifact pair of water walking boots.

Assuming that shallow water is implemented: all giants should use the shallow water behavior of only wetting their feet even when walking through deep water. Also, fire giants can wade through lava, which acts mostly the same.

More harassment effects: the chance of respawning the Wizard is a fixed 1/7 and the rest of the effects are equal probability. Multiple non-duplicate effects may happen at the same time.

• Unchanged: curse items, summon nasties, vaguely nervous, aggravate monsters
• Darken much of the level, and extinguish, destroy, or cut the remaining lifetime of light sources. Maybe also blind the player.
• Drop boulders around the upstairs, or iron bars.
• Spawn pools centered on the player, with sea monsters in them.
• Spawn lava around the level, not under the player.
• Create a hole or pit under the player.
• Create a stinking cloud centered on the player.
• If you are carrying the Amulet, it teleports to a random square on your level or the level below.
• If you are carrying the Amulet, it (and possibly any fake Amulets) becomes heavier. This either lasts until the next harassment triggers, or is permanent but the total amount of weight added to the amulet has some sort of cap on it. This is interesting primarily because it targets carry cap, which is a valuable end-game resource.
• A variety of powerful player debuffs such as dealing half damage, taking double damage, blocking HP or Pw regeneration, super hunger, etc.

You take fire damage from standing next to lava, levitating or flying over it, or walking on it. Damage from standing next to it is (number of adjacent tiles)d8, whereas being over or on lava has rather higher damage. Fire resistance does not prevent this damage completely.