How about a deployable for the sniper, instead of cloaking. Make it a cardboard cutout, almost like a target for target practice. I'm thinking of archery practice here, but with something like a standup cardboard cutout you see for movies and such. But allow the sniper to choose the image on the cutout.
He could use it to draw fire away so he could snipe, to trick enemies into thinking theres an engy sitting behind that turret, getting people to twitch and waste a burst of fire as they round a corner, etc. And he could help out teammates by planting it in a snipers LOS, giving the teammates additional cover against sniper fire. All without being overpowering.
Let pyros ignite it, give it pretty low HP, and a sweet shredding animation when a heavy unleashes on it.
Plus I think it would look sufficiently cheesy and light hearted to be included in TF2.
For a pyro I like the fire extinguisher idea as a Fire Axe replacement. Primary fire he swings it, doing far less damage than the fire axe, secondary fire have it spew a cloud of smoke, that can be used as cover. Give it low ammo, a very short linger time, let it putout friendly fires, and boost fire axe damage.
I also think he should have some kind of area denial weapon, something that causes a cloud that slowly drains health, but all pyros are immune. Help him funnel enemies into an ambush.
I also thought of a "Wire Cutter" for demo. Allow it to sabotage enemy equipment and stickies, so that they detonate when used, or in proximity of an enemy. Allow it to override friendly Sappers, so he can work in cunjunction with an enemy spy to cause havok. Also gives a tricky alternative (although dangerous) to just blasting away enemy stickies.
