I can play all of them, but I'll comment on the ones I play(or played) at least semi actively. I won't comment on DRG since they've been working on it's rework for some time now so they probably already have it nearing completion. (that said, I hope they don't screw it up.)

NIN:
  • Revert the mug<->Trick Attack change (this means party damage on trick, personal on mug)
  • Make ten-chi-jin (the ability) change to Meisui while Suiton is active

SAM:
  • Bring back Kaiten
  • Upgrade Ikishoten to Ogi-Namikiri after activation, then Kaeshi: Namikiri after that
  • Make Shoha II an upgrade to Shoha
  • Revert Senei and Guren back to a 50 Kenki cost

Reaper: (I don't know, I'd have to think a fair bit.)

Monk: (I don't play it enough to actually know what I'd want changed, but the rework they put out for it is solid. Maybe make bootshine a positional again?)

BRD:
  • Bring back the kit interaction with DoTs (potency on sidewinder and shadowbite)
  • If not bringing DoT interaction back and removing them at this point, make the support more potent, and lean into the support aspect more with the 2 free slots you've created. Foe's Requiem's return would be a great start

BLM:
  • Make Fire IV and Blizzard IV take the same slot on the hotbar (meaning Fire IV while under Astral Fire, Blizzard IV while under Umbral Ice)
  • Lock the timer on Umbral Ice when activating Umbral Soul, so that we don't lose our gauge while there's a long transition in any fight.

SMN:
Just make require more thought than the amount I need to do nothing else. It's as simple as it can possibly get short of playing itself. I don't care if it's bringing back DoTs or shaking up it's core gameplay but something would be nice.