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).

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.

Cavemen receive a spellbook of fireball as their crowning gift, which they can always cast with 100% success rate. Flavor-wise, this ties into “discovering fire”.

Make fire giants (possibly only in the Valkyrie quest, possibly others) spellcasters who can only cast a fireball spell. Or, to avoid a flavor issue with giants being not very smart to be good at spells, flavor it as something other than a spell (it could also be implemented as a new attack type).

Artifact wand of fire which casts fireballs instead of rays. (Where a normal wand would zap a ray, this instead casts the Basic form of the fireball spell without needing Pw or knowledge of the spell.)

Artifact magic candle (requires magic candles to have been implemented) which casts a version of the fireball spell (at Basic level, with a single explosion) when invoked.

Make fireballs like old-school D&D: the explosion isn’t a certain radius; instead it has a defined volume, and will expand out from its origin point via random breadth-first search. This makes it dangerous to use in confined areas.

The fireball spell’s unadvanced form creates a single fire explosion at a targetable square, rather than a ray that explodes, so you can cast it at squares that aren’t directly aligned with you.