I've been tossing this idea around in my head for a while, and I think it could help with some of the weird choices that need to be made in loadouts, and make things more flexible.
Remove the "heavy carrier" and "light carrier" options completely. Instead of only two options for a possible load-out, add primary, secondary, and GL ammo as selectable loadout options in addition to other items. This would basically allow you to build your "vest" out however you want.
The UI could look a lot like the current weapon and accessories one. Open up a "Vest" option like you would a firearm, select the level of protection (Heavy, Light, None), and then equip pouches to it. This would let you choose the number of primary and secondary magazines to carry, and balance it with launcher ammo and grenades. It would allow players to choose to carry more launcher ammo or grenades at the expense of ammo.
It could also help balance some weird choices with ammo that isn't a standard magazine. For example a 1-point primary ammo pouch may hold 2 standard magazines but only one drum magazine.
The primary problem I see with this is adding too much complexity to the loadout screen, and adding more time to people getting ready before the match starts.
As a bonus, the system could pretty easy be used to display customized vest layouts for players reflecting what they have chosen to carry. That would be pretty cool.