Mod written by GitHub: Our Lord And Savio rGabe Newell / NexusMods: OB4MA
This mod makes it so that you can use anything that might normally have a level restriction on it at any level.
I haven't thoroughly tested this for progression throughout the story, but it should simply just remove the level requirements while increasing stats/damage.
I tested how this works with game generated loot by loading up an OP 8 character in UVHM, then switching to an OP 8 character in TVHM in the same lobby.  The loot downgraded it's power to that of level 50 loot and not that of 80.