I've always wanted a basic looking handgun with average stats, that could either use the RMB to morph into a grapplehook/hookshot type thing, or just have the grapple shoot right on clicking the RMB.
More specifically, I'd like it to be a square pistol that has basic damage, ROF, etc etc. But when you right click, it does an animation where the sides of the pistol come out into a crossbow shape, the slide (assuming there is a slide) would come back into the open position, and the grapple or whatever it would be comes out of the barrel. And then it would shoot and hook onto something similar to other addons out there, but with hopefully a limited range to avoid across the map flying, and maybe even a more innovated grapple action, like swinging instead of just gravitating towards the hook, or something.
Edit: It might not be a realistic military issue idea. But it seems like something MI6 might manufacture if it were a more over the top series :D