It wouldn't be much work at all...they have glue to turn rare masks into legendary, they have drives to turn rare sparrows legendary. They've already done it in other aspects of the game.
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They would have to literally double all the rare guns and armour in the game because rares and legendaries are separate items. There probably isnt enough space for it but the idea is excellent
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Well I don't know anything about programming really but couldn't they like have things organized in groups and whatever they throw in that group is automatically classified as "rare" or "legendary" so all it would take is a copy paste from the rare group to the legendary? But yes, it's quite a good idea, would allow a lot more variety.