I like your basic theory. I would also like to see, HQ synths gaining more Materia Slots. For example, Iron Gladius using 2 Deaspected Crystals would make a weapon with 2 Materia Slots. The Iron Gladius +3 Made with with 2 Deaspected crystals would be 5 Materia Slots.

Either that or Certain gear has a certain amount of slots when created. HQ versions increase that number of Materia Slots by the HQ value. This would create the need for different sets of gear, for example

Vintage Haubergeon - 1 Slots Static
Templars Haubergeon - 2 Slots Static