Sinister Reign - More directly reward fast kill speeds.
I'd make it so the party leader could select the maximum number of opponents: 3,6 or all 9. I would expand the choice pool to reflect the number of opponents beaten within the time allowed.
HTBF - Add Oseem.
The drops wouldn't even matter. I'm sure we'd all line up to punch that jerk in the nuts. The fight could borrow some of the quirky, comical effects used in the add-on scenarios. And he could lock/remove gear during the course of the fight as a gimmick.
QoL Walk of Echoes - Add a vendor NPC.
QoL Emporox - Merit and Job Point overflow would automatically be sent to this NPC. Even if Job Points were a 1:1 conversion, something is better than nothing.
Add a use for Job Points after Master - 500 buys a Revitalizer (not the "Super" one).
Restore King spawns - Add ilvl 145 versions that drop a few Voidwalker synthesis materials.
Popping NMs is convenient but not mysterious or exciting. These drops would make it questionably worth it, but still fun to do on a lark.
Nerf BLU - I would change the colors in BLU AF, Relic and Empy gears so they match those in the Armada set.
Fix BST - Undo what they did.
Buffs, gear and opponent design do not match a desire for Master + Pet melee.
Find a use for Charm. Maybe as a Stun. Maybe as an Absorb-TP.
Fix THF - Replace Maximum Treasure Hunter Value gifts with Inquartata.
Evasion should mean something, especially on this job. Since ilvl gear is cross class, evasion is not only worthless but also diluted.
Fix RDM - Spell Adjustments.
Frazzle spells would be non-elemental.
Haste II would max out magical haste.
I would add a non-elemental Full Dispel. If a spell would have to be eliminated to make room for this, goodbye Deodorize.
Fix BRD - Spell Adjustments.
Threnodies would behave like Ninjutsu. They would still lower resistance, but would also do damage and magic burst.
Other useless spells could be replaced to do things like Gambit or Rayke.
Fix MNK - Gift and Weapon Skill Adjustments.
Like with THF, MNK's epic HP is diluted by cross class ilvl gear.
Guarding Skill Bonus would become Hand-to-Hand Skill.
Martial Arts Effect would become Critical Hit Damage.
Counter Effect would become HP +10%.
Shijin Spiral would get very low duration Amnesia, so it could be used like a Stun without preventing mages from also trying to Stun.
Victory Smite would be adjusted to work exactly like Chant du Cygne. The fTP would be lowered and then allowed to carry through all hits. The % chance to critical hit could remain lower than Chant du Cygne's to compensate for the additional hit.
Asuran Fists would be changed to behave like Resolution. Testing would be needed to ensure other options remain relevant, but I want to have a valid reason to Tarutaru-flip on a regular basis.
Make our Inventories Great Again - Adjustments to Porter Slips, Storage and Furniture.
Porter Slips need to go somehow.
Storage would no longer be based on furniture. It would just be another 80 slots to do as you please.
Furniture would be stored on the Mog House Moogle itself, somehow.
I know there would be more things. But I think these options would be worth investigating. I don't think they would require dramatic resources and would gain far more than pain the developers, the player base or the life of the game itself.