What I would like would be something that wouldn't require any change to the loot system and everything. But it would require data stocking on materias.

It would be a "consigne" where you can put savage stuff, get a ticket to use another one (for instance, you loot a torso, your main is a caster, you put that caster torso in that place, get a ticket against which you "borrow" a Melee torso. Your caster torso is temporarily unavailable, but it still has its materia ; when you give back the melee torso it also keeps its materias, waiting for you to pick it back). You could register sets like glamour ones, and switch when you switch jobs. That way you avoid the classic feels bad moment "my main is 530, I'm bored I'll try Drg but it's 510 and will do a lot less damage for several weeks".

Since the pieces you need for BiS change from class to class, this system would require you to buy at least one piece of each tomestone stuff, and loot a piece of each savage stuff, to be fully BiS on every job. But that would be just one time (one tomestone torso, one savage loot only), that's still miles better than having to loot / buy several pieces for each job like now (because going 510 against 530 people isn't really something to enjoy...).