I saw that the devs were open to suggestions with respect to this, so I had a thought - what if players could use the cache more than once to create more than one legendary for each time they visited and completed the sanctum.
But in the interest of balance, and in keeping with the game’s spirit of ‘risk-reward’, each time after the first they open the cache to put something new in, it could spawn a ‘paradox’ that they have to fight. Losing means losing all the legendary items they created, winning means another chance to create a new legendary.
Just a thought