I don't much like the OP's idea, but some of the replies have been interesting.

A crafter that could make materia, and convert hidden slots to guaranteed, could be interesting.

Also maybe change one materia into a different type. Being able to pick exactly what materia you were changing too might be a bit too OP, so it could be at random (tho same level of material obv, not changing a IV into a II or anything), with a small chance that it levels up? Anything to put a stop to that Mutamix nonsense