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.

For bow specifically:
Suggestions for ammo types

Emp arrow: this arrow could be shot at a vehicle to temporarily shut it down. For heli me ir would make them hover instead of fall.

Spot arrow: the spot arrow could be shot at the ground to ping all enemies in a 36m radius around the arrow.

Sonic arrow: this would be an arrow that would have some self propulsion to allow it to go further for sniping.

Explosive arrow: it would explode 5 seconds after firing and do 1/2 a c4 in damage.

Piercing arrow; a normal arrow that can pierce 3 enemies doing 50% less damage per pierce.