It's always fascinated me that a literal flamethrower doesn't produce or spend heat. I'd probably have it generate heat, gains increasing depending on the number of enemies it hits. It would do a lot on big pull trash clears and probably be a niche on fights with packs of adds but that's fine because it's a capstone ability, it's meant to be something that can be fancy. There's also a sweet irony in the fact that the whole Machinist role is movement taxed and our only ability that literally requires us to stand still and can fail to do anything if the enemy is moved out of it is our weakest.