I'm not sure how hard it would be to do this, or if it's even possible, but it would be really fun for a survival game, or a mission where you have to break down your gun. A mod that would allow you to take parts of a gun (lets say M4) and make the gun. Instead of just finding an M4 in a survival game, finding pieces of it (Stock, reciever, rail, etc.) and putting the gun together, using a custom GUI. And then being able to break down the gun into individual pieces to fit into a backpack.
-EDIT-
After thinking about it a bit more, it would most likely only work with guns made specifically for this mod, seeing as you would have to have different pictures and models for each individual part of the gun.