No need for different gear sets. Just have a toggle where elemental resistances is located. Have a toggle for resistance, weapon enhancements,could even have a toggle for alternate skill hotbars there like set1= hotbar1-8, set2=different hotbar1-8,etc.
The gear would just handle caps on all those parameters with higher gear yielding higher caps.
Examples:
Adamant armor set= 1000 total resistance cap
Adamant Lance= 1000 total enhancement cap
Put all 1000 in fire and you can't increase different elements further.
The question is, what kind of system would govern this?
Swappable materia sets? Merit system?Crafts? Special spiritbond? The game has no horizontal progression, so multiple gear sets is out of the question.