Now, I've talked about this a ton of times (even got posts dating back to the CTA), but I really think you get shafted on pistol ammo. Four mags with a Heavy Carrier and two mags without a carrier is pretty lackluster considering you'd get three mags without any ammo rig at all in Ins2014, and seven with a Chest Carrier. I loved running around with just a sidearm in Insurgency: Source, but it's just not viable in Sandstorm unless you've got a 31-round Glock (which I still think is pretty BS, honestly).
While a general ammo increase for sidearms would be nice, I personally want an extra gear attachment that would give you more ammo for your sidearm; probably like 3-4 mags or maybe even more (it's not like pistol mags are hard to carry lmao). This secondary/pistol ammo pouch would cost one or two supply and would use the "Accessory" attachment (used for the Gas Mask), which I think creates an interesting counter to the extra ammo, and doesn't make it a "no-brainer" attachment. If you want more ammo for your pistol, you'll lose out on the benefits of the Gas Mask, which includes no coughing in smoke and traversing through Chemical Gas without taking damage.
Those downsides will probably make most players still prefer the Gas Mask, and the pouch will be mostly used by us pistol mains, Marksmen and possibly Breachers. Considering that pistols aren't as good as primary weapons, I don't think that additional sidearm ammo could be unbalanced, to begin with (except for, again, the Glock with its insane ammo capacity).