like rewards from raids nightfalls, a similar drop rate as legendaries and exotics?

Only as a second or third item drop in addition to the main drop. No one would want it in place of gear. The main use would be for PoE, weekly heroic and NF when there is a burn modifier. It would be a nice solution to pass some hard content when you don't have the right elemental, helps with the chicken and egg problem (you need good gear to pass content that awards good gear). A timer / cool down would be a must. Similar to this, a PVE only ammo pack for armor piercing, explosive rounds, maybe a few others would be a fun reward that keeps you playing longer. Since it would be consumable, it wouldn't replace any guns. It might make some interesting synergies for weapons that couldn't normally roll certain perks - which might make it tough to implement since there is probably a reason that some guns (ex exotics) have carefully controlled perks. Would love to see explosive rounds or armor piercing on my thunderlord though.