I'm not totally against the idea of the ability to change itemization, but implementing a system like the aetherial gear at endgame isn't how to go about that because it is, again, random. And if you've ever played Diablo 3, you know how unfun it can be to farm the same item you already have over and over just to try and get that perfect roll. (RIP that perfect Sacred Harvester I dismantled)

Perhaps an expansion of the materia system would suffice for this, but honestly, materia alone is probably enough. It's easier for SE to balance the classes when they don't have to take into account a bunch of player decisions.