Ammo type unlockables that diversify what you can do with weapons. For example: AT for the barret or snipers allowing vehicle damage with guns at the cost of ammo capacity or fire rate, Ammo on pistols or SMGs that does 10% less damage but fires 15% faster. Maybe the opposite for that too, increasing the damage it deals at the cost of firing a bit slower. Something that can unlock different playstyles for each weapon, and vastly increase the replay-ability of weapons without being a different attachment. Could have different types of ammo also regenerate quicker or slower depending on how powerful it is.