I wouldn’t mind something found deep in military installations, that is just a giant heavy cannon-thing that’d be useful for base-defense when the game gets to the point where you’re holding off big groups attacking your home…
You’d carry this massive thing/drag it to where you want to set it up, (a)pply it and it stands there on its feet immobile. Like a bigger, heavier turret than we have now.

Boom … makes a crater, fries the squares around it, sends things close-by flying a short distance.
Make it slow to “charge” (requires a USP, or even better something rarer like plutonium, or hydrogen), slow to fire, so you can’t just boom the crap out of everything that comes.
Just a fun ‘omg what did I find’ piece of high-tech, higher-tier loot.