I would like to see Tenacity be useful since having defense and damage as well as increased HP recovery is pretty neat, but the formula makes it significantly weaker and it would just create an odd problem in particular. Direct Hit is always needed for that extra boost in damage, but if Tenacity was particularly strong then Determination would lose out as a result. I don't know how they could make more melds viable but I always just called it an illusion of choice anyway. If we just had a substat menu that we could freely change at any time based on class, and assign a certain amount of points to it(like SHB and EW relics), we'd mainly just pick whatever increases damage.

The melds are just sort of a relic of an older time but one that isn't the easiest to phase out without impacting a fair few things like hunts and market value, along with older materia still being needed for random things like ARR relics. The problem is the most we can customize is just damage output by the end of the day, and anything that doesn't give the best damage increases isn't really worth it since kits in general are more than equipped.

That being said, having Tenacity not be a meme anymore would actually be a really welcome sight. I would like if Skill Speed was primarily based on preference(go fast or go slower, whatever is more comfortable), but I don't see how they could change it too much without just swapping meta materia around or completely reworking certain substats entirely. That'd be the only real way to provide a more measurable fix, because as it is not there isn't too much outside of like, going for a doctoral degree in math for the .1% maximization.