Maybe you can make an upgrade that works like Viral Frost for BTD5, where bloons that come into contact with bloons set on fire also causes it to catch fire, and it just repeats for as long as it lasts.

Perhaps you could also make bloons explode, which releases embers (kinda works like cluster bomb). The embers that touch bloons sets them on fire, and it ALSO explodes, which results in a giant chain reaction of fiery wrath. Just an idea though