Different Ammo Types.
What if you were able to by different kinds of gun ammo from the store by the magazine/clip/round? Things like hollowpoint, incindiary, and maybe even explosive rounds. These wouldn't be unlimited, of course. You would have to buy them from the store, where it has its own section. You would still have unlimited normal ammo with this addition, and it wouldn't replace normal ammo when you purchase an ammo type. You know how you can put ammo in their own spots on your vest? When you buy this special ammo, it would appear on those vest slots automatically, like how buying a pistol puts it in the pistol slot automatically. Different ammo types would diversify playstyles and make matches more interesting.