Remove the reserve shooter from the class entirely and replace it with something that isn't obnoxious gimmicky selfish bullshit
Degreaser getting the flamethrower's pre-buffed airblast
Flaregun raising weapon switch time or actually any change at all that would make it not be a viable axtinguisher replacement/alternative
Give airblast some kind of falloff so that scraping a scout with the tail end of the blast doesn't give the same kind of blast as destroying them at point blank with it
Fix the double jump airblast airstall bug
Fix the class's weight/mass so that they can't surf rockets literally better than scouts can into my medics and demomen
Buff the escape plan so that the powerjack isn't literally absolutely better in every single way other than a fraction of a second of rollout time
Third degree can't crit or be crit boosted
Any buffs or nerfs that you shoehorn into the flame damage from flamethrowers themselves will only make the class even more obnoxious to play against. The status quo of flame and afterburn damage is the way it is for very finnicky balance reasons. It's one of the few aspects of the class that valve actually managed to get right after tinkering with it for long enough.