Even if there is some sort of "History" with the materia fusion process, it will obviously cap out at a certain point and have limitations to what type of materia can be used on certain peices of gear.

Lets say we can progresivly creat a stronger and stronger attack materia...We could just as easilly speculate that upping the stats on said materia may increase the "Level requirements" to use that materia. Lets say there are 5 tiers of materia.

Minor
Lesser
Normal
Strong
Radiant

Lets say Minor gives you... +5-10 attack when your materia goes beyond that cap it become a lesser materia of strength 10-20 attack. Minor Materia can be used in any weapon lvl 1~+, lesser materia can be used in any weapon lvl 10~+ and in such varying tiers until you hit Radiant Materia of Strength equipable only on Rank 50 items.

Thats one way to assume how the stats will be controled on a growth scale, obviously a oversimplified vision but it's the mechanics of how it "Could" work. For all we know Materia may have no lvl restriction and just have max caps on what the stats can go up too. I highly doubt we will see legacy materia with +999Str and +999 acc.