FFXI went one step further with special effects like dual wielding, I'm sure you meant that too but I thought it would sell the system more than mentioning VIT

. It was a fun part of FFXI, also worth mentioning abilities themselves had stats sometimes- not just combining stuff.
I would love to see them inspired with like Diablo 3 legendary horadric cube storage system as well (maybe you can slot one affix regardless if you equipped the right spells). Some thoughts with Witcher 3 mutagens + ability like affix system.
Opportunity to want to collect everything as well. There could be some game changing affix (in the sense it restructure what you originally approached a kit). More than just + basic stat, maybe certain ice and fire spells allow you to slot black mage like light mechanics (that umbral stuff).