I think the issue stems down to, that all materia should be useful in one manner or another but for the most part perhaps 40-50% of it could be considered useless if we look at it from the grade 4 standpoint.

32-34(Attributes, potency, HP,)

22-24(Resist, DoL/DoH)

One thing I don't get is what's the point of elemental resist on the secondary tool, seems really odd to have such a thing that can only be added onto your secondary tool when it doesn't seem like it would be useful at all. Even though gear can have DoL/DoH stats on it we're unable to add such stats onto it even though the gains themselves are somewhat small, most you can get froma grade 4 is +6 lol compared to the +16-20 you can get from attributes.

There are prolly a lot of oversights with this system but the main one is it's rich mans game concept, it doesn't really act as a time sink since the only ones willing to take a risk are the rich, with how risky the business is people would rather spam for rare/ex gear that has guaranteed stats on it rather then spends millions at a chance for something that's only slightly better. Because of that people also aren't willing to add things like stat resist since it's all situational so as stated above nearly half the materia is considered useless and only there to act as duds for what people really want.

If it came down to stat resist I think they should change it up a bit. With the elemental resist have it where each grade has 1% ele dmg of x element taken down on it. So grade 1 would be 1% with it's 1-3 resist and Grade 4 would be 4% with it's 11-15 resist. For a Paladin they would be able to give themselves a 16% dmg taken down towards one element which would give them an edge while tanking a magic heavy mob of a specific element. To balance it out a tad just make it where cap for ele dmg taken down is 25% so a person couldn't stack on ele resist for 3 pieces of gear and give themselves a 60% dmg down for one element lol.

As for status resist they could add it where each grade reduces the ailments duration by 2 seconds, so a grade 4 would reduce a status ailment by 8 seconds, For this the cap would be 50% of the base duration for a cap of 30 seconds. so a status ailment that last 1 min would only last 30 seconds, though based on the flow of battle it's hard to say how effective it would be since people are usually on top of healing ailments. But the idea would at least be a starting point to make them more appealing, in a fast paced battle if you did happen to have a piece of gear with the -30 second duration on ailments and got hit with one that's base was 30 seconds, it would only last 15 seconds which may wear off faster then the mage that cure based on the current situation of the battle.

The other thing they could do is also add a % mod to gain immunity to a status ailment upon wearing off whether it be naturally or through meds/magic. For that it could be a bit higher perhaps 5% at grade 1 and 20% for a grade 4 with a cap of 50%. So if you were hit with a status ailment and recovered from it with the capped rate you would have a 50% chance to gain immunity from being hit by said ailment for 1 minute.

Maybe in the same sense an even smaller mod could be added to the ele resist to make them more appealing by addinga .5% absorbtion rate when hit for a cap of 5-10%. 2% for grade 4, with 5 of the grade 4 ele resist equipped you would hit the cap and havea decent absorbtion rate which could help during the flow of battle lol.
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As for inventory issues as stated above there may come a time when a new grade of materia is added so how should one be able to deal with their inventory load as well as the increased variables where perhaps the 51+ gear will drop from II-V or III-V which is still a lot of materia to think about lol. With that in mind I think that when materia hits the next stage all previous materia should only have one variable, so right now since we have up to grade 4, 1-3 should only have one variable to help ease the stress on the inventory load, then when we are able to obtain grade 5 materia the variables would be removed from grade 4 and added to grade 5. Though based on the success rate of attaching multiple grade 4 materia I do ponder if perhaps they'll only stick with grade 4 since when you look at it from the icon point of view it looks like a perfect orb of materia so perhaps they don't have any plans to go higher then 4 and the lv cap will just allow you to hit a higher attribute/stat cap so that you can attach more grade 4 materia without hitting the cap.

I hope materia takes a bigger reform though, i'd like to play around with it, but it's only purpose is to achieve the lucky achievment and the crown, honestly I'll prolly never attempt to meld anything past one once I manage, if I ever managed to get that achievement since it just isn't fun thinking about spending mils just to lose it over luck lol.