Pyro axe tweak: with alt-fire, you can swing either axe in a wide arc, causing the head to fly off of it wherever you aim. It's a slow attack, and it does more damage (or ascends to higher critical levels, or something) the farther it flies before hitting the enemy. However, you can only do it once per resupply, and afterwards you're left with just the axe's handle, which still swings at normal speed but only does bat damage and now gets no random crits. Call it a desperation ranged attack.
Syringe gun replacement: the Big Needle. Sprays a powerful general anesthetic over a range a bit longer and narrower than that of a flamethrower. A liquid, it arcs, and has a minor effect where it pools on the ground. It does small amounts of damage, but the muscle relaxant causes an intense speed debuff and prevents jumping and melee attacks for a brief period. Small "clip" size and longish reload time, probably enough for the effect to wear off. Doesn't actually have to be a big needle, since if you think about it there's no injection happening here. Might as well just be a canister of some barbiturate or other.
Fist replacement: hurly gloves. Only does damage to buildings and other Heavies; against other classes, instead of doing damage, you pick them up, and have one second before they break free during which to throw them at another nearby enemy or a wall. The lighter the enemy, the farther they can go. Under various circumstances you can indeed get mini-crit or critical throws. I feel like I already made up this new unlockable.
Dispenser replacement: munitions cart. Replenishes and heals slower than a regular dispenser and has less health, but it automatically moves (pretty quickly, too) towards allies low on ammo. Cannot move across no-build areas, but it can jump a little bit.
Sentry replacement: repeater. Leveling it up increases its health (lower than normal) and ammo capacity (which is very small at level 1 and nigh-bottomless at level 3) but not its firepower; the weapons it fires are auto-tracking versions of the weapons held by allies standing within range of it (think like dispenser range). So if it's just the Engineer, it fires shotgun shots and pistol shots. If a Soldier comes by, it also fires rockets; if two Soldiers are there, and one has the Direct Hit, it fires both kinds of rocket. If a Heavy is there, it fires whatever minigun the Heavy has. A Pyro nearby means it shoots flames; if the Pyro has the Backburner, the flames get backcrits. It might also get flares. Snipers give it rifle bullets (it shoots a fully-charged body shot every ten seconds or so) or Huntsman arrows, and maybe Jarate. A nearby Medic gives it the power to shoot needles, but Blutsauger needles do not restore its health. Non-weapons bestow no powers; Buff Banners, Sandviches, Mediguns, and the like have no effect, and no melee weapon (even the Sandman) gives it any powers. Firing all these at once means it will tend to run out of ammunition very rapidly. If no allies are nearby, it just shoots pistol shots.
Sentry replacement: Decoy sentry. Tracks enemies and pretends to fire, with realistic sound effects and fake bullets and rockets, so that from a distance it looks like the real thing. Doesn't actually do anything, though I guess they'd be useful for reaching high places and blocking enemy movements. They cost 25 metal a pop plus 25 more for each upgrade level (for a total of 75 for a level 3) and you can build as many as you want. Can't be repaired, only upgraded. Maybe make them cost more and make the bullets/explosions still have full knockback (but no damage).