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I had to think hard to remember exactly how materia was used all those years ago now ^^;
I think I would reserve such a concept perhaps for possibly strengthening individual ULTRA RARE Evoliths through the use of The Trial of the Magians system or even special Quests.
These Ultra Rare Evoliths themselves could start fairly powerful in terms of stats, but then be gradually upgraded through a series of Trials or Quest that could ultimately set them apart from all other Evoliths. They could even gain multiple stats per one Evolith, making them really desirable in comparison to other Evoliths. Perhaps only one Evolith of this kind would be able to be fitted per gear/weapon slot as a result. Here's a quote from the FF wiki describing how Materia leveled up that really sounds like our TotM's system:
"Winning battles earns Ability Points, which are Experience Points for Materia. Just like characters, Materia levels up when it gains enough AP. Some Materia allow access to stronger commands when leveled up, others allow the user to use the granted command more times, others give a stat bonus that grows larger, and so forth. When Materia reached its highest level, it is "mastered", and a second Materia with the same powers is born. Thus, with patience and enough leveling up, even one-of-a-kind Materia could be reproduced infinitely." (read more about FF Materia)
In any case, the concept would fit nicely ^^
