I've pissed a lot of people off in tf2mix by playing pyro. Here's how I would change the pyro to make him viable in 6v6.
PYRO 2.0
--------
slot 1: flamethrowers
slot 2: Shotguns
slot 3: flareguns.
slot 4: melee.
Health: 150
Speed: 107% (same as the medic)
-Flare gun and detonator are now combined.
-Detonator/flaregun jumping now make you go a little further and higher.
-New boston basher style weapon for the pyro. Ignites people instead of making them bleed. This can be used to build uber.
MY REASONING BEHIND THESE DRASTIC CHANGES:
1) Pyros currently get to decide between the shotgun, the flaregun, the reserve shooter, and the detonator. I feel like each of these covers one of the classes weaknesses, but you can only choose one of them so you're always left really weak in some other aspect. For example, Pippy the Pyro chooses the shotgun. He's now good at cleaning up and has midrange combat capabilities but he lacks mobility because he didnt choose the detonator.
2) The pyro in highlander is kinda like the passive scout in 6v6 because he helps defend the combo from roamers, spies, and scouts with balls. If pyro was buffed then teams could switch out a scout for a pyro.
3) The pyro should always be able to have a shotgun equipped so that scouts can't completely wreck pyros like they can right now. A pyro with a shotgun has a good chance at winning a 1v1 with a scout.
4) Pyros should always be able to have the flaregun/detonator combo thing equipped so that they can do burst damage, and get those lucky long range across the map crit flares. Pyros should always be able to detonator jump because they're going to need it to be effective in a lot of 6v6 situations and maps.