If you had the power to add one feature no questions asked, what would it be?

[ThaCa]

Expand icon system and adjust it's requirements.

Mentor Crown becomes different variants of Watering Can depending on the mentor, PvP mentor gets its own requirements.
Crown and similar are made in different visuals for Ultimate, Savage,(maybe even Extreme) clears. PvP gets their own icon corresponding to ranked tier, top 100 or higher placements.
Crafters and Gatherers that have finished Expert recipes and/or Relics get their own fancy icons (max level desynth?).
Maybe even add it for stuff like Blue Mage accomplishments and whatnot.

Or just tie fancier stuff to Titles.

Watch as the chaos unfolds.

[Renalt]

RP isn't just dancing and emote spamming in a venue/saying a couple of lines in character and then getting paid gil for it at the end of the night.

[Allegor]

Butt/pec slider for male PCs

Honestly I didn't put it as top priority but unironically agree. The perfectly straight, flat plank of a chest we have is pathetic, meanwhile, games like Lost Ark:

[Packetdancer]

Redo the dang loot system.

If someone's inventory is full, as soon as loot is available, toss a warning up for them so they can throw something out. Ideally, have a 'holding space' for like... 3-4 items, where if your inventory is full it goes into that space until you have discarded something, at which point, the next item in the 'holding space' falls into your inventory. (I've had friends whose inventory was full after a map night lose out on things they wanted. And I know someone who missed out on an ultimate clear totem because their inventory was full and they'd forgotten.)

If you cannot roll on something ("You have already obtained one <X> for the week.") then just automatically pass on it. If it's something with a Unique flag and I already have it in my inventory, pass there, too! You cannot open a coffer when bound by duty, you cannot access your chocobo saddlebag and retainers, there's literally no way to resolve that short of doing "Pass" anyway. (Or throwing out the unique thing and then... rolling the identical thing, solely to deny it to someone else. Which is just sort of a jerk move and doesn't really need to be supported.)

And for the love of the Twelve, give me a 'Greed All' and 'Pass All' button! And ideally a "remember for the rest of this instance" checkbox; I really, really, really never mean to roll on anything in Sastasha. I promise you this. Let me just tell the game that and never see a loot dialogue in there again.

[Gemina]

Hairstyles to glamour plates.

[Hopebot]

I don't know why everyone settles for more pre-chosen dye colors. Just put a color wheel in the game for everything that can have its color change. all values from 0-255, hair, eyes, face markings, tattoos, gear, furniture, all on a color wheel (skin can stay as it is).\

Also as somebody said, yes, an overhaul on the glamour system, not having to keep gear, and I would personally love a portable glamour thing.

(Also also, super personal preference but I'd like a real and proper made fox race in the game, Miqo'te don't really do it for me....)

And then Have Alliance raid loot 1 per boss when it's new, not one per entire run.

....Actually there's a lot I'd want.

[BaconBits]

Hairstyles to glamour plates.

Jandellaine bout to be working overtime lol.

[Atelier-Bagur]

Style Lock/Unrestricted glamour. Game becomes 100% better

[Connor]

If we’re allowing literally impossible suggestions, personal housing for each char in their island sanctuary instance

[BaconBits]

Style Lock/Unrestricted glamour. Game becomes 100% better

The nanosecond that happens I'm just going to turn every one of my combat glams into an Assassin's Creed outfit.