A gold and silver skull to be exact.
The gold skull gives enemies the upgraded weapon variants. I think this could really make Halo 5’s campaign one of the most difficult, as opposed to one of the easiest (in my opinion). Normal difficulty will only give some enemies superior weapons, legendary will give all enemies superior weapons. When they die, they drop the regular weapon. (This could also be used for vehicles as well).
The silver skull would override the last part if you have the gold skull activated, allowing you to pick up the more advanced weaponry. If you don’t have the gold skull active, then enemies will have regular weapons, but you pick up superior weapons.
What do you think?