The Arsonist is a really weird merc and would make more sense as an outlaw since normally mercenaries represent elite troops and not random criminals or jobbers. So make Arsonists a weaker high pop outlaw and fill the missing Indian merc grenadier space at the tavern and Indian home city with a Mysorean Rocketeer that launches rockets with the same animation as the fire thrower and switches to a musket or pistol in close range.
I agree, if/when they rework the Indians, the developers should add in Mysorean rockets, both to boost their artillery options and to credit the Indians for developing military rocketry. The British Congreve Rockets were modeled from the ones they encountered in the Mysorean Wars. At the time, those rockets gave quite a scare to the redcoats when they first encountered them.