Loose bolts - Engi's wrench
Allows the engi to carry multiple buildings at once. 3 max metals per second lost in the way. Hitting 0 max metal still allows the engi to carry the buildings. The max metal capacity is regained via the passing time. A dispenser accelerates this max metal capacity regaining process. It only works when the engi is deploying the Wrench, otherwise it functions the same as when you are playing normal. Having a shotgun in his hands results in the engi not loosing the metal.
How to select a building you are about to put is done by PDA style HUD, after hitting the Reload key the selection menu appears, hitting the corresponding number will change the building you are about to build. Hitting the main fire key will build the buildings like usual.

I.M.P destructive device
Sticky launcher, a building damaged by this I.M.P sticky launcher loses the ability to communicate with the engineer, he can't observe the HUD indicating the building's states in the left corner. Engi has to get close to inspect the building's health etc, simply putting the cursor on the building from far away won't tell you the specifics without the connections lost by the weapon. It requires a complete rebuilding to solve the situation. Simply hitting them with the wrench won't fix it. Putting the building in a box then reconstructing the building will fix the building.
Downside is -30% damage against the buildings.

Bell-type Lure Sapper
This sapper is throwable. This sapper has no functionality associated with destroying the buildings, but it creates a false alert. It creates a noise of the building getting sapped forever. Not exactly forever but 1 minute. After 10 seconds the sapping noises can't be heard by non-engis. Well, because you can imagine how the weapon would be irritating in a non intended way, specifically when you are not even an engi. Everyone's buildings are faked to be getting sapped. Well, engi can diagnose the state of the sentry even when he is not near the buildings, so there must be some wireless connections of the sort going on in the system. So the spy can transact the communications. It affects all engis in the server. It's pretty destructive as it can mask the real sappers. Especially when the engi is not being able to see the buildings directly.

About the R.E.D Sapper
Make the tele re-fill the charge before allowing the engi to teleport. Specifically, just counter the minimum seconds required for the teleporters of the corresponding level. level 1 tele should take 10 seconds before allowing for the spy to teleport. Level 3 would allow the spy to travel in under 3 seconds. So this makes the weapon less spammy after destroying the teleporters I think.

Loyal Overdo - Engi's exotic shotgun (Or simply Over Enginerd)
The stats start from +30% spread, 20% greater damage falloff(artificial damage debuff via landing point distance) 60% sluggish, slower reload. Staying near the dispenser regains the true potential of such over-engineered mechanical pieces. It surpasses the stock shotgun in the peak performance, but it degrades over time. The max ammo (not in the clip) is 5). totaling 11 ammo with the ammo inside the clip. The level of the dispenser dynamically changes how the weapon looks. Going to level 3 buffs the max ammo from 3 to 6. Otherwise the spread and falloff buff is depending sorely on how long you've been near the dispenser. The ammo can be replenished only from the dispenser, the ammo source is limited to the dispenser built by the user of this weapon himself. The weapon's final stats are debatable but, maybe 30% better faraway damage and stock spread.

Reverse engineering device (Sapper)
Sapping the tele exit automatically takes you to the tele entrance. Or sapped tele still working as a 2 way tele only for the spy would be suited. The teleportation can tele-frag people outside of the spawn. It doesn't have to be a sapper even, it can be a passive effect. In that case, notify the engi with sounds that the security is bleached. Or it should be a long lasting sapper, lasting 30 seconds. Anyone in the span is able to take the tele exit to get to the tele entrance (probably) placed right next to the spawn site. But it only encourages spawn camping but nothing else, so this later part of the concept is never going to be okay. Not being capable of breaking the tele yet disabling the ability of the tele itself. Sapped tele functions as a 2-way tele only for the spies having this weapon in the loadout.

Yes, I will clarify the details.
This sapper can't break the teleporters by itself, sapped tele still functions as a 2-way tele only for spies having this weapon. The spies can tele-frag players. The players in the enemy team can't take the tele. The sapper creates very iconic noises, the sapper is visually very easily recognizable.

Weapon maintenance wrench
Hitting friendly players with this wrench lubricate or adjust the weapons the players are carrying, resulting in far better cycling or a handling of the gun overall. Thus, giving the teammates slightly better weapon stats. It works like it's in MvM. Modifying the base stats of the gun the players are carrying. But it's only limited to certain weapons, alas, few weapons are not equally buffed. At least not in the same way. It usually starts from a better cycling speed. +15% reloading speed and +10% firing speed. It lasts permanently, resupplying at the respawn cabin will remove the effect. It's on the weapon so, dropping the weapon even for once, the buff is removed from the player. It gives the weapons similar visual effects to the KIllstreak glow. The wrench is something oily.

Dispenser modifying wrench
Allows the dispenser to give the effects above yet it's time-based. Shortly after firing the gun for once, it starts reducing the performance.

Decency Thanksliver - Demoman's crafting tool
His backpack-based item. His secondary taunt is overwritten with a pairing taunt much like the Rock-Paper-Scissors taunt. He gives 9 mercenaries a consumable tool depending on the class type he is paired with. The weapon is actually swapped with the previous item the player had, so anyone willing to par take the action should be on their will. Anyone can take the offer yet there is nothing to force other players. Alcohol based gas passer alike for pyro, Bonk cocktail (Soda item) for a scout, gyrojet for engi. Something like that. After the consumption the original weapon in the owner's loadout is loaded, or the item the users previously had in their corresponding slot. Bonk soda slows the scout but as long as he is moving he can ignore the incoming bullet damages, but explosive damage still does the thing on the scout. He can't shoot the guns but he can swing his bat. It's pretty much a meme.

Audio clue - Spy's ringing ears. Uses the revolver slot.
Standing still for 2 seconds allows the spy to see the fogs through the wall, but the fogs are never in the class's shape. They are vague and yet to tell the spy the location of any enemies, it even tells you the invisible spy's location. As it's rather audio based. Hearing gun sounds nearby hinders this effect. Even flamethrowers disable this through-the-wall effect. Hearing loud noises prevent the spy from hearing the audio clues. Only works against moving players. Melee is ignored. The see-through effect only works against players through the wall, directly visible players are never highlighted. It's not even an outline. It's just a huge chunk of fog which is telling the location of the enemies, as I said. All classes share the fog visual effect, meaning it's impossible to tell which class he, or they are approaching.

Not a weapon overhaul yet a concept that should be applied to all my sniperrifle concepts. The damages done to the sniper aiming through the scope is registered as not having a damage fall off, or turn it into min-crits yet give him the equivalent buff of 35% damage resistance to neutralize the debuff, I know it won't be the exact 0% of damage difference. There should be some "unjust" coming from the nerf-then-buff method. In that case, don't register it as a mini-crits, just use the "no-falloff" part of the effect whatsoever.

Baby rattle - scout's melee
Basically a mackerel reskin
Mommy Milk - scout's drinkable (In a baby milk bottle)
Increases the clip capacity of scout's primary up to 69, increases his reserve capacity to 69 too. Gives him 100% ammo refil after the buff. It lasts 16 seconds. After 16 seconds, the scout loses the ability to store ammo for 7 seconds.

Fall in one mighty gadget can be named "Sampler" or "Blood Test" instead. Also whether it should be applied to the patient and the medic or medic alone is arguable. Since it's not always a good thing to be forced to use a different weapon. For the sake, the strong weapon is focused more on being a weapon created by mixing all the good stats of their class's weapons. The weapon the patient and the medic gets is according to the patient's class. If the medic ubers Heavy, the medic can use the minigun for a temporary use.

Fall in one mighty gadget - Medigun
Prepare 9 non-accessible weapons for each classes, the weapon is a mishmash of all the class-based weapons. The medigun charges up in 30 seconds. The medigun's effect is applied like Vaccinator, as long as it's applied the medic doesn't have to stick together. The medigun allows the user to use the strong weapon.

Soldier's banner - Heavy loading arsenal
Pressing the assigned reload key will instead swap the weapon with his other primaries. You can't reload the primary while the banner is active but spamming R gives the player the rocket launcher he demands. There is an order of what weapon you cycle, there is a fixed pattern instead it being random. The system is like picking up a weapon from the ground, the soldier doesn't have to own the weapon because it's pulled out of the system not the player's inventory.