I think the old "igniting touch" melee effect would be a very good match for a warlock exotic.
(Defeating burning [scorched] enemies makes them explode [mini ignition], enemies damaged by this explosion are burned)
I think it could be a full out ignition when you defeat a scorched target, but it would have to have some sort of cooldown / charging system (for example every 4th / 5th scorched enemy you kill ignites and more powerful enemies give more progress or when you scorch a target, a charge meter similar to Manticore would start to build and when it's full, killing a scorched target cause an ignition or aplying scorch causes ignition or st like that). And then additionally your ignitions spread scorch (the effect of ember of char).
I'm trying to avoid any kind of cool down.
This is meant to give us bottom tree Dawnblade back, which was able to chain more explosions off of kills with explosions.