Tanks simply don't need tenacity due to how much mitigation value they already bring, cooldowns are so strong, passive inbuilt defensive values ect.
I think to make a useful "utility" melds would actually be to separate them from offensive melds.

Personally I think we should remove Direct hit... at least from tanks and healers, Skill speed and spell speed can be merged for the sake of Paladin more then anything (Dark knight too in aoe), So that you got offensive meld slots which are Haste, Crit, Det, which in that i think Haste stats should be on par with crit builds, just less RNG dependent but more skill based as you've got a faster GCD to manage.

For "utility" melds we could go wild. (If they were separate from offensive materia)
Tank: One that increases max Hp, Recovery and your healing done, one that increases passive mitigative values, one that could increase cooldowns effectiveness and durations slightly. (Only defensive)
Healer: One that could increase Healing & Mp recovery, One that Increases your cooldowns effectiveness and durations (again only defensive/supportive), One that can increase shielding/mitigation.

Obviously this idea could have more done with it, I think it would be interesting if we could augment certain skills with materia, some that could make a tanks raid wides or target mits stronger for off tanks ect.

The current system is a boring gil sink I think something needs to be done to actually make melding interesting, I doubt that though.