Just a few gadgets and such that would be useful or generally fun to play with.
First, portable cameras. The system is already in place, so the ability to place a camera on the floor or something, and then access a screen in your inventory, and cycle through set up cameras would make moving about an enemy base undetected a bit easier. I would prefer being able to throw the camera around corners and such, and then actually move it around, but I'm not sure how practical that is given the engine.
Smoke grenades, and/or flash bangs. Other than the FX, it would be as simple as a debuff to the accuracy of affected characters. Also, with the smoke grenade, it could be interesting if you had thermal goggles, which would negate the debuffs while in smoke, so you could throw it directly at your feet if being crowded.