WoW's glamour catalog.

Some damn respect for inventory concerns that doesn't default to removing major reason for leveling a gatherer before looking at all the bloat that makes up how merely redyed gear at the same ilvl (brown Striking tunic vs. otherwise identical green Scouting tunic, etc.) or Materia are handled...

A better graphics engine.

A better physics engine. Barring that, at least a functional Z/height-axis (was removed in ARR). Ideally, go further with it, such as by allowing for certain mechanics to be leapt over, or even having levitate/launch/juggle mechanics.

Certain skills being reducible to VFX without model movement and being therefore castable even in the middle of another animation. (Call them Techniques instead of Weaponskills, Spells, or Abilities, if you like.)

Higher server polling rates, and greater polish by which to remove the wonkily timed outliers among snapshotted skills, delayed effects or effect activation, or skills which, by nature of how they're coded, have less effective duration than they are balanced around.

More data centers. (Oceania's a great start. What about one for East Coast US or, say, Brazil, especially is we ever get Spanish or Portuguese localization?)

Dynamic movement abilities, whereby one can control the destination through mid-animation commands instead of it being entirely snapshotted. (Imagine a Gyoten allowing you to instead charge, to a maximum of 20 yalms, to the far side of your enemy, instead of just the nearest outer edge of its targeting circle.)

Sub-hitboxes on larger units, such as the legs of a colossi (where we instead find ourselves typically swinging at thin air between their 10-yalms-apart legs).

More realistic turning for mobs.

Dynamic/tracked animations, such as would cause a Heavy Shot to actually aim at its target, instead of merely directly at the angle of player facing.

AoE animations going off nearer to their damage application.

Skills that look like cleaves actually cleaving (and exactly as widely as they appear to cleave).

Latency subtraction/recuperation on animation locks.

Better use of uniquely-(or just well-) animated in-game storytelling, as opposed to just cutscenes.

More varied animations and camerawork during cutscenes, as opposed to mostly static shots and generic emotes.

Better support for ground-targeting (cast-on-release).

Targeting schema and/or functional combat macros (with at least the first contextually actionable line therein being queueable).

The ability to click through non-applicable units (based on current filter or queued cast).

(Dynamic/applicable) target snapping for mouse or joystick.

A greater number of available difficulties (though not so rewarding as to make previously irrelevant content, at new difficulties among 4-man content, obligatory in preparation for traditionally upper-level content like 8-mans outside of the weekly tomes grind-loop).

...