The dev team has fallen into the bad habit of most MMO dev teams of only paying attention to the formula and abandoning old systems. Things i was hopeful would one day be updated but never have been...

-Chocobo's system hasnt seen any development in years.
-Older Fates like Odin and Behemoth left in the past instead of being updated every expansion and keeping cool social aspects around for New and old players when they pop.
-The Gold Saucer barely sees new additions, they dont have to all be major, but even those are years between something new.
--Chocobo racing for example is basically just an abandoned side hobby now, dont even get new tracks in new areas let alone anything else cool to spice it up.
-Guildhests still sitting there being an abandoned system instead of figuring out something do with with it, Tutorials for mechanics was my idea before 7.1
-Variant having 3 dungeons but no daily roulette? i still dont get that...
-Hunting Logs still being the same system it always has been is pretty sad, I had an idea years ago now that a new Hunting Log UI that looks like an actual book would be cool, that adds some new monsters for every job and every expansion, just for completionist purposes would be fine, But i also though a page that tracks A and S spawn locations if you've killed one there would be pretty handy and then an SS page that records hints on how to spawn SS's but alas, same as its always been.

Thinking more as i typed these...
Sightseeing log actually has regressed, While i dont mind having an easy one for the average player, the Riddles of ARR was pretty cool and it would have been nice to atleast have "Basic Sightseeing and Advanced Sightseeing log" No to mention no rewards...

Underwater is essentially a flop, used as a exploration gimmick in Stormblood, I would have liked to see it actually turn into something more interesting than what it is.

Fate's in general have failed to evolve at all, the addition of Bicolour gems was nice, but mostly just a grind, I would really have liked to see cooler Fate Chains. I still think old multiplayer boss fates from ARR and HW should be Red on the map and reward Hunt tokens/nuts which would make people actually show up for them, and then some cool Fate Chain could actually lead to red Fates... with maybe super rare Purple Fates that are basically modern Odins etc.

Years I've been banging on about Multiplayer MSQ cutecenes and scenarios, everyone hates "This Scenario must be undertaken alone" or whatever it says, not just for Veterans but new players, I dunno who doesnt think it would be great to watch the Party leaders Cutscenes with them and even join them in their solo Scenarios, hell it would be cool to see the other players standing in the background too but not required tho... Just let people play the "MMO" together...

Theres so much more i could go on about, bigger things and minor things that should be addressed but are quite clearly never going to be.