Pyro shotgun replacement: Fire extinguisher. Shoot it at an ally, it extinguishes any flames on 'em. Shoot it at an enemy, they're slowed. Shoot it longer, they're slowed more and their vision is obscured. Maybe defuses sticky bombs. Good Backburner companion.
Minor Heavy buff: make it so he can't be blocked by enemies, except for other Heavies, maybe Spies (for spychecking purposes), Ubercharged enemies, and buildings. That is, rather than being stopped or going through them, he pushes them backwards. While Ubercharged, only another Ubercharge can block him, and his knockback is reduced further. Also, his melee weapons now have a knockback effect.
Minigun buff: make it so (some of?) the bullets pass through the target nearest you and also hit guys hiding behind him. Add an achievement for killing two guys with a single bullet.
Heavy shotgun replacement: Cossack throwing axes. They are axes and you throw them. No need to worry about reloading; they arc, though, and the range is limited, and of course a compression blast can have fun with them.
Medigun replacement (or tweak): you can spend your ubercharge at any time in order to get partial damage reduction. Charges more slowly.
Medigun replacement: longer range than regular Medigun. You can click on allies and enemies. Health is siphoned from most recently targeted enemy to most recently targeted enemy. Effect ends when enemy leaves the ally's range. Enemy and ally remain "marked" for up to ten seconds after leaving range. Ally cannot be overhealed by this method. On ubercharge: marked ally (or self, if mark has expired) siphons health from all enemies in range. Enemies can die from health siphoning. Somewhat "hands-off" Medic style but still requires consciousness of healing duties. Possibly requires boost to maximum HP, since Medic will be going forth into the face of danger more often.
Flamethrower replacement: big flamethrower. Very large tank carries more ammunition, but slows movement speed to Soldier level. Range and damage increased considerably, but cone of fire is narrower and the flames themselves travel more slowly. Made of brass, with detail work and a bunch of conspicuous gauges and hoses. Compression blast does not extinguish allies, and it has a smaller range (both in time and space) for effectively working, but if you blast an ignited enemy, they take heavy damage and the burning effect is renewed.
Soldier shotgun replacement: a machine gun. No tricks, no fancy effects, just shoots bullets fast and reloads quickly. This has been the subject of conjecture before.