My old suggestions on unlocks in H3 that expand already existing fire mechanic (similarly how H2 unlocks expanded on the electrocution mechanic)
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Bottle with gasoline and canister with gasoline. Both are used to create puddles - at any time you can hit the prompt and leak all the gasoline on the floor. The bottle is unlockable and concealable, while canisters can be found on the level, in garages and utility rooms (could be added to the H2016 and H2 maps too), but obviously isn’t concealable. The canister either creates a bigger puddle or can be used twice. Puddles can be ignited just like puddles from the oil barrel - via cigarette, electrocution device, EMP or The Big One, etc. All NPCs who stand in the puddle will catch on fire, which results in accident kill.
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Lighter, matchsticks (x3). Can be used via prompt near the gasoline puddle or thrown at the puddle to ignite it. Lighter can be used multiple times; each matchstick can be used only once, but there are three in each matchbox. Possibly, to balance it, not suspicious?
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ICA Sparking Phone - just another phone that will attract NPCs and will drop a spark when answered, which could ignite a puddle.
Fire Extinguisher explosions and blown water tanks will stop the fire.