Imagine an activity like the Forges or Reckoning where at the start you can place or "bet" any weapon or armor piece of your choice.
If you fail the activity, the item is lost.
If you succeed, the item gets either rerolled (for items that are already legendary) or upgraded into a legendary gun with a set of legendary perks and the appropriate amount of slots. Oh, and all the stats, even the hidden ones, get little bumps too upon upgrading (the degree and extend of said bumps could be randomized).
[spoiler]I don't assume that this pitch goes anywhere but dreaming is nice.[/spoiler]
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would be cool if: -you find a blue you like with perks you like. -take it to the gunsmith and pay legendary shards to make it upgradeable. -use the weapon, kill X amount of X enemy type, or kill X amount of guardians in the crucible. -once complete take weapon back to the gunsmith and choose which perk you want to add to the weapon using upgrade modules. perks like out law and range finder cost 3, perks like firmly planted cost 1.