Spy disguise kit
300% damage resistance when disguised
When pointed out that he is a spy via the voice command, he explodes.

Demoman two in one secondary
Very uniquely shaped sticky. Absorbs bullet type damages done by teammates.
When this sticky absorbs bullets of teammates, it fragments on explosion.
The damage dealt this way is given contributions to the original sender of the bullets.
The damage is concerned, not actual bullet that gets reused. Every 20 damage starts making one fragment. There would be less than 20 damage fragments, for individual teammates. Like a flower pollinating. The enemy gunfires are ignored for this feature, but instead, it explodes the bomb. This sticky absorbs 50 damage before exploding, flare explodes this sticky in one hit guaranteed. When manually detonated by the enemy gunfire, it deals damage against the enemy who tried to damage it. The bystanders are not damaged. This sticky is detonated by pressing R to open a menu, then pressing the number key like a spy's disguise kit. Each sticky is assigned a number. You can see the number through the walls. Stickies are shown in X-ray. The sticky's absorbed bullet damages are shown in a form of color. It's white, but turns red. Can lay down 9 bombs at the same time. But the bombs have to be separated for enough distance from eachother. So you can't place stickies over the same place. The minimal distance between stickies are 4 meter. And the sticky is automatically deleted when it's inside the radius of a previously laid down sticky.

Last resort - as previously mentioned

I'm assuming the engi can shoot twice before anything happens, and later engi can fire once and it goes off. Thinking it doesn't actually deal damage to sentry.
But what about eating it off. Maybe 2 rounds against any buildings to make them explode. And it should be in a small time frame. Like during 10 seconds.

Hammer - pistol of engi
1 third of fire rate compared to stock. 40 (base) damage each. Holds 9 rounds.
It has a heat meter. When the heat meter is high, the accuracy and damage is reduced. Firing 6 rounds continuously and it reaches the max heat meter. The damage drops to half.
The heat meter is reduced over time.
Only carries 50 or 60 reserve ammo.

Hammer ii - melee of engi
Repair efficiency 15% increase. 40% damage increase.
Can't level up regardless, but able to add metal to max. Friendly engi can level it up instead.
The amount of metal you add per hit is reduced by 30%.
Everytime you hit your buildings, you roll dice. There is a meter indicating the amount of buff you get for the next hit. The max meter indicates 30% increase of damage for next hit. Hitting opponent doesn't roll the dice but reset the meter to 0. The range is between 10~30%. Atleast 10% is added. It's 40% base damage increase plus roll damage. 119 damage max. No other randomness on top of it.
Maybe it's better being max 70% dice buff, ranging from -30% to 70%. Without the base 40% buff.

Fragile shaped glass - Wrench
Everytime you spend time wrenching your buildings, 2 damage is added per hit for the next attack that successfully lands. The max ramp up of 109 damage. Requiring 22 hits for the buildings. The wrenching that actually counts are when engi spends metal. Repairing or adding ammo. Regardless, no damage randomness.

Last resort
Shooting your own sentry with this thing for 3 times will make it explode.
The explosion's radius and damage is corresponding to the level of the buildings.
Only when the virtual damage is exceeding 50, it counts as legitimately shooting the buildings with intents.
The attributes is 60% more damage against enemy buildings.
This thing only loads 5 rounds.
This thing has 1 reserve ammo. When the reserve ammo is 0, colliding with ammo box does not make engi take it, unless the metal or ammo for other weapons is low.
The pellets penetrate enemies indefinitely.
When the buildings are destroyed via PDA, it causes an explosion of the identical damage and range. The explosion visual effect and the damage radius is different for the explosion. The damage radius is significantly wider than the explosion visual effect for the clarity.

Final destination
Firing this engineer's shotgun grants 0.2 seconds invincibility against any attacks, not a simple damage reduction.
A pump action shotgun similar to stock in design, but loads one round.
Damage is the same as normal shotgun.
When reloading, hitting the right-click at the exact moment in the middle of animation gives +1 extra round to be loaded. Max of 3 rounds. The window of 0.2 seconds for the extra reload. The timing for the extra reload is entirely client sided, not for the invincibility.
Second option for the extra reload would be to
When negating more than 60 damage, extra round is added.
Would be cool but the damage is not consistent, sometimes a rocket deals 54 damage. And the consistency would be lost. Maybe 30 damage? Any pistol or smg like weapons, when they deal more than 30 damage per hit it would be triggering at any moment. Spammable high damage weapons like minigun when the damage is boosted with gimmicks, they could be used to trigger the weapon. But 30 could be sufficient. The threshold of 30 is used to see if it's a big enough damage to trigger the instant extra one ammo in the clip.

When the floaty-mode is activated that is.

Acid rain. Scout's exotic shotgun.
Or a Toxic rainfall.

The angle is not the camera, only relative to the positions of the players.

Firing against opponents from vertical angle gives better damage, how angled it is is directly affecting the damage. When fired from right above, and assuming all the rounds hit, it deals 150 damage. When fired horizontally, it deals only 50 damage at max.
It has a floaty mode switch. Basically, it just switches the descending speed to be really slow. Allowing the scout to glide. At it, it's without costs. It just allows the scout to toggle the ability. When firing at the angle of 45, the max damage should be 100. When the scout is on ground, the damage is fixed at 60.
It has the basic round capacity of 6, everything else is in accordance with the stock.
Maybe he better get an unusual-like effect of clouds appearing around him. In a way not blocking the view from other's perspective too much. And a rain effect that actually falls under the projectile category, non-damaging visual-only projectile. The rain effect follows the center of the invisible projectile. And the scout creates this marker-projectile, many of them to simulate the rain.

This antenna effect is only applied when the scout is holding the melee.
The line between the medic and the original patient is made, getting inside the line heals the scout. But the line is straight and the beam is not a very accurate visualization of the line. The point in the line scout is the closes should be made accountable, and the distance should be measured. So the scout doesn't have to be directly exposed to the line to heal. Getting near the line and the scout is healed. Taking medkits doesn't take away the medkit in the ground, it just heals scout and gives the scout 15 seconds until being able to take medkits again.
The pollination weapon concept I posted back then contained the idea of replacing the medkit with a better one, like pollinating flowers. But instead, above system of not actually taking medkits added with invisible extra medkit that forces to be taken by someone even when the HP is at max, or just extra invisible medkit. It would be hard to limit the medkit from spawning while the replacement is on. Otherwise an orange colored medkit to remind the honey would be nice.

Give me that, or E antenna.
Getting inbetween medic and the patient heals you.
Antenna for scout. Slot of the bat. Heals at 30% rate, or 100% normal.
Taking medkits doesn't take away the medkit, but still heals scout. The scout can't heal from medkits for 15 seconds, not 10 seconds. When taking medkits, the medkit is visually replaced with a orange colored one only for the scout. Or this visual cue is not necessary.