If I myself were DMing, I would allow Frostballs to start magical 'cold fires', which had the same effect as normal fires. Acidballs would start a kind of magical 'acid chain reaction' in which acid was magically produced and reproduced (a real mess if ever there was one.) Sonicballs would generate sonic blasts that would slowly escalate, and so on.
I would do this, if I and my players agreed that fire should be a Chain Reaction or an Infinity Weapon. It would be necessary, in each case, to determine what was immune to each 'element' (as stone withstands fire, other things withstand other elements.)
In an Infinity Scenario, where fire is the most powerful, the players must realize that their enemies WILL use fire on them (why wouldn't they?)
But then again, fire would be powerful. An enemy hit and enveloped in flame would have to make a DC check versus the damage or be rendered helpless, unable to even act to put out the fire by rolling, and anything in that square or adjacent squares would have to make a Dodge Save versus DC 11 or be set alight also (unattended items would receive no save.)
The target would take 2d6 damage per round, every round, minimum. If the target had greek fire on him, 2d6 extra damage per round. If the person was so unfortunate as to be in metal armor, 1d6 the first round, then 2d6 per round additional damage, and this would continue indefinitely so long as the armor was worn.
Any magical items destroyed would cause a burst of magical energy, which would inflict a yet addition 1d6 to 5d6 points of damage to all in that square and adjoining squares (depending on the power of the item.) Magical items would be VERY hard to destroy with even magical fire, much less ordinary fire, but if they were destroyed the consequences would be truly bad for the person wearing them. Artifacts and relics would be immune to fire.
In the Infinity Scenario, a shootout using fire spells or greek fire in the Middle of Town would be a real bad idea. Few spells have much fire dousing capacity (a Waterball, if it caused an Infinity effect of blasts of water, would help, but destroy things with the water blast.) Medieval methods would be hard pressed to stop the fire once it really started in on the wooden buildings.
This is a case of Everyone Involved Is Arrested and Charged With Multiple Murder Counts, Multiple Arson Counts, and properly loss claims in the tens of thousands of gold pieces. Can we say the Town's Local Version of 'Ivid's Endless Death' for the offenders?
In a woodland, the resulting forest fire is going to cause EVERY intelligent creature for miles who cared about the place to go after the pyros with a fury.
In a dungeon, the heat, smoke, and lack of oxygen is likely to make for a TPK for all sides involved, and creatures completely uninvolved in the fracas.
Yeah, in the Infinite Fire scenario, it's a party for all involved. The invitation is mandatory. Just don't count on the (utterly infuriated) town cleric throwing Cure Light Wounds on those 3rd degree burns!
Play with fire? You may win with this tactic. But you will, in the end, get burned.
