I was actually thinking about this awhile ago. If you could only get it from scavenging, I think it would've been more balanced. I think having the weapons locker made it too op, since you could grab 2 (or 4?) at once. I liked the ultimate grim reaper from DR3. (BFG, Laser Sword) I'm surely open to some "fun" weapons sprinkled about. I know DR2 had some wacky weapons as well, though. Having more "down to earth" combo weapons would also be great. Why do any scavenging for weapons when I can pull 8 700-round miniguns out of a bottomless locker and not worry about anything? I'm a pretty big balance guy so I hate getting stuff for free.
There wouldn't be a point where comboing on the fly would be completely free, which makes sure the game is still balanced even at level 50. Leveling up would decrease the amount of duct tape and time taken (just like DR3), however even once you reached max level it would still cost duct tape. If you didn't have enough duct tape, you wouldn't be able to combo the weapon right then and there, and would need to take the items back to a maintenance room where you could combo them for free. Comboing would take a determined amount of time and leave you vulnerable. Once you reached an early level somewhere between 5-10, you would unlock the ability to combo anywhere in the world. Maintenance rooms would exist and function like DR2. One way I thought they could do this, was to introduce a "duct tape" currency\material. But I also think comboing on the fly was a handy feature, but it needs to be balanced. I really liked the maintenance rooms in DR2, because even now I still enter them not knowing what's inside, usually 1-2 pairs of items enough for a couple combo weapons.