New artifact Staff of Agni, a quarterstaff with the following fire-based properties:

• Can be zapped if its enchantment is positive for the same effect as casting a spell of fireball.
• Invoking it sets its enchantment to 3 + your skill in attack spells. (No effect if the enchantment is already higher than this.)
• Grants fire resistance when wielded.
• Deals double fire damage in melee like Fire Brand.
• Acts like a robe for reducing spellcasting failure, but only for the spell of fireball (and fire storm, if that is a separate spell).

Fire Brand gives you extreme vulnerability to cold damage, making you take quadruple damage from it. If you have cold resistance, this only reduces it to triple damage.

Frost Brand works the same way with fire and fire resistance.

Make it impossible to have fire and cold resistance intrinsically at the same time. If you eat a corpse that gains you one, you lose the other. Things like crowning that give you both now do not give either (unless perhaps you have neither, in which case you get a random one).

Extrinsic fire and cold resistance are not affected. This makes it valuable to have those extrinsics from a ring, underused colors of dragon armor, or an artifact.

All polymorphable monsters that currently resist both fire and cold must have one of those resistances taken away; otherwise you could have both at once by polymorphing into them.

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.

Extrinsic sources of acid resistance should protect your weapon from passive corrosion attacks, and extrinsic sources of fire resistance protect your armor from taking fire damage.

Artifact ring that provides a cocktail of basic resistances all at once (fire, cold, shock, poison, maybe sleep, maybe acid). This is interesting primarily because it is very useful early in the game and then becomes progressively less useful later on as the hero gets intrinsic resistances.

Could be named “The Ring of Everything Resistance” or “The Chromatic Ring”.

Water demons should resist cold instead of (or in addition to) fire.

Non-fire-resistant creatures have a very small chance of spontaneously combusting in Gehennom.