Turn it into a movement class.
So far we have 3 offensive classes:
Scout (movement based)
Medic (DM and movement based)
Spy (Loosely movement based since he can get by sentries)
4 defensive classes:
Soldier
Demo
Heavy
Engie
Classic O v D CTF focuses on fragging classes vs movement classes, a very compelling dynamic IMO. Pyro does not fit into this. Therefore, either make him defensive focused (then it would be 5 classes vs 3) or . . .
Make Pyro a 4th offensive class that has a jetpack. (Replaces the flamethrower) No afterburn beyond level 1, and no flamethrower. IC Cannon cool because it can be used for starting movement with the jetpack.
If nothing else, remove burn completely. Rage inducing mechanic as FDA said and it's just not fun. RAGE INDUCING MECHANIC! REMOVE IT FOREVER! NOBODY WILL MISS IT! HOLY SHIT REMOVE REMOVE REMOVE THAT SHIT!