The attachments section would be renamed to modifications, and you would now have eight ways to modify your gun. Here is your breakdown.
Optics. You get the idea. Attach an optic to the top of your gun. On top of this, there should be a remove optic option and an M231 "sights" option. That kind of optic modification doesn't exist right now, and it would be nice to have.
Muzzle. This is a new section, with old attachments. This is anything attached to the end of the barrel. Muzzle brake, suppressor, you get it.
Barrel is also a "new" section. This is physical change to the type of barrel your gun uses. You can change the length of your barrel, or suppress it, like how a honey badger does. I could also see smoothbore being a thing?
Underbarrel is the same for the most part.
Magazine is another new section that has some old attachments, like the AA12 20rd drum. This is separate from ammo. You can change your mag to house more or less rounds which effects ballistics and reload time.
Ammo is unchanged. However, unlike before, you can have a 22 vector with 50 rounds. Or 10.
Stock is a new one. You can get rid of the stock, retract it, give a gun a full stock or a retractable stock, anything like that.
Other is old, but some attachments are moved to sections where old attachments now fit. Canted sights and the ballistics tracker stays, of course. The Glock Stock is an example of a change, that would go in stock. So would the boom stock. For the Tommy Gun, the magazines currently in other would be moved to the new magazines category. Of course, make sure ballistics are changed, due to the new potential chaotic combos you can make.
In review, you have these eight modification types: Optics, Muzzle, Barrel, Underbarrel, Magazine, Stock, and Other. This would spawn unbelievable abominations of guns. You would be able to make any gun just right for you. It should stay in the testing phase for a while, as this could make some crazy overpowered guns. If done correctly, though, I could see this being a super cool update. It could possibly revolutionise gameplay, which hasn't been done for a while. If any of this could be implemented, please separate the barrel attachments from muzzle attachments. Just that alone could make some fun guns. Muzzle booster + Long Barrel? Oil Can + Short Barrel? Long Suppressed Barrel + Muzzle Brake?
This kind of update could be fun. It could create new guns for very particular people who want a gun just right for them, but could also be implemented in a way that doesn't create overpowered guns.
Along with this, after getting 10,000 kills with a gun, you could change the material/reflectance/colour of all parts of your gun. Every part and mesh can have colour individually changed for the most part, allowing for you to make your gun look just right. Want to make it 100% neon? You can do that now, if you want. Want to fine tune every individual part to make the gun look as realistic as possible? You can do that now, too. Side note here, you cannot buy this. You need 10,000 kills to do this. People are relying too much on credits right now, rather than actually using guns.
In conclusion, this update would bring an insane overhaul to gun modification, both with physical modifications and skins/appearance. Keep it in testing for a while before going forward if something like this does happen, of course. I'd like to see something like this go super well.