2 ideas:
1) Somewhat related to coflropter's idea, this machine is one you build by your buildings. The building itself is like almost a satelite shaped/umbrella shaped thing that can extend out and automatically detect projectiles, like rockets, grenades, and stickies around itself. The "Umbrella" itself will then do things accordingly to what kind of explosive it is. If it's a rocket or grenade in the air, it will just move in front of the rockets/grenades trajectory. If it's a grenade or sticky on the floor, it will cover the stickies/grenades as to minimilize damage on the other buildings. The umbrella itself has around 400 hp, so that it can take a lot of fire, and buy time for engies to go out to battle.
Something like that, except the dome is more curved in, so its like a semi-sphere. The only question is to make it replace what weapon. But idk.
2) A superpowered wrench that 1-hit kos any enemy buildings. On the flip side, this will only give buildings +15 instead of +25. Imo it gives a great incentive to Medics to uber engies. And it'd be pretty funny.