Make the class consistent and not rely on ping and dumb luck, mostly.
Make the Manmelter actually do something without the Phlog equipped
A new, interesting secondary with a cool concept like when the Wrangler or Gunboats were introduced that changed the metagame of their respective classes
Or a primary weapon where the sole purpose is Medic protection (probably a shitty concept in play but I've thought it would be cool)
Make the Axtinguisher usable again
Define Pyro's fucking role in the lineup once and for all

Turn the bad and underpowered current gimmick mechanics into lightning gun mechanics bring back the holy third please volvo.

also fix your fucking game holy christ