I was thinking it could be interesting to be able to have a slight upgrade to 5-star equipment.
It could work in a way such that it's definitely costly like you'd think an upgrade to a maxed equipment piece would be, but doesn't require a recipe. I think it would fill the role of "endgame" items in preparation for the new missions and such, so you don't have to throttle a ton of money and resources into buying variants.

For the upgrades themselves, each item would have 2 or 3 of it's strong traits selected and you could improve on those. For example, the Cold Iron Vanquisher: Increasing the bonus damage to Undead, the base damage or the speed of the weapon. Maybe even a slight particle effect to note the type of change you made for the weapon.
Another idea would instead be miscellaneous upgrades, something you would see much more as just a cool bonus. Some examples I could think off the top of my head are a weapon dropping a guaranteed heart after every X amount of enemies killed, armor increasing your defense power of every type besides the one for the damage type you have received after getting hit for X seconds. Maybe even giving a limit of one of your pieces of equipment a cool active skill

Dunno if this will sound dumb or not to most people, i thought it was just a fun idea