I just want to start off with this is not something that is necessary, however I think this could benefit the game in the long run by making all kinds of materia useful and not only 2 (this is mostly the situation right now).
The materia we have the in-game right now are not always going to be useful, especially if you want to do savage raid and/or probably the new criterion dungeon. right now the most useful materia is direct hit and critical hit materia, sometimes people opt to go for determination, however mostly people go for the first two.
skill-speed, spell-speed, tenacity and piety are not being used by the vast majority of the players in the game, you could argue that black mages go for spell-speed, but that is 1 class out of 19 classes (excluding blue mage), you could also make the argument that black mages probably don't go for it that much either, because of all the quick cast spells they have (ex swiftcast, triplecast, but also include things like foul).
I am obviously speaking for myself but also some other people I know that feels the choice of what to meld is not very interesting, as 9/10 times it will be crit or direct hit, I don't know what the solution is to this however.
I know that piety and tenacity was introduced as a replacement for parry and accuracy, however right now as the game is with the values of each substat, there is very little incentive to take those stats over crit, direct hit or determination as a healer and tank (with their respective stats)
I still think the current version of materia we have is completely serviceable, I just think that if there is a way to keep the same kind of class balance as we have now while making the other materia feel more in-line with the other ones, maybe via some random probability.
For example have piety not increase the regeneration rate of your healing over time, maybe change it so you have a chance to have an extra mini-heal or something along those lines, obviously not at the full potency, imaging have two 800 potency heals back to back through 1 cast.
Feel free to add to this post if there is anything you'd want, some ideas you have, or if you just want to discuss this.
Edit: 1. I do acknowledge that I may have exaggerated a bit in my post, however it does seem like everywhere you look people say, just meld critical strikes and direct hit, instead of going for any of the other materia, if I am wrong about this that's fine I'll can
admit when I'm wrong ^^
2. the accuracy materia was changed with determination as the stat, not piety, piety has been there for a long time.