Tenacity in it's current state is not really great for a ton of reasons, A lot of it has to do with the lackluster defensive benefits outtop of defensives not being as important, plus making a "defensive set" for progression runs would likely just be more of a hassle then it currently is. I also don't think just increasing it's flat damage to be the best stat would be the best solution, at that point it can just be removed lol.
How could we "Improve" Tenacity
-Tenacity Could Increase Targets HP more.
-Tenacity Could Increase the effectiveness of Defensive cooldowns (Like Rampart, could go from 20% to 25% if you put in max Tenacity as a example).
-Increase the effectiveness of the current passive effects.
All Of this would certainly improve that stat, I doubt it would be as useful as crit/DH, so maybe tuning bosses to actually hit harder with autos could also help encourage the stats use, My main hope is that melding has a little choice in it (even if its the illusion of "choice" at the high end) with at least having the option be viable and have actual upsides.
Other Melding issues.
-The lack of having "sets" such as three different melding sets on a same gear, this can fix issues where one job favours another stat, or you could make a more "defensive" set and a offensive set for tanks.
-Skillspeed/spellspeed, Tanks such as Paladin/Dark Knight suffer from having Skillspeed more then any other jobs, due to having spell attacks... Personally I think both stats should be merged into "recastspeed" which would help jobs that use Phys/magic.
-Piety kind of weak? I heard this stat was also kind of bad.
Maybe theirs other issues with melding or it's fine? But I just thought that maybe improving the melding stats to be more functional would at least add some semblance of "customisation" (make skill speed paladin would be slightly less of a meme!)