I noticed that the idea of the napalm missile was floated about on the discussion board for awhile, but it doesn't seem to have gone anywhere nor has it been acknowledged on the trello board. I personally think a weapon that deals little damage but deliberately starts a massive number of fires would shake up the game considerably and place more emphasis on the oft-forgotten fire extinguishers.
In addition to this idea, a different type of Ammo Factory that produces an incendiary round would be a good way to add to the versatility of cannons and railguns. None of the physical weaponry should discriminate between ammunition types if more types should be added. When an incendiary round is fired, it has a smaller overall damage output spread over a much wider area, with an increased chance of starting fires.
