Things from 1.0 era that could return

[Enkidoh]

Path Companions, and hell, the Path of the Twelve itself - I dislike what it became as the Scions of the Seventh Dawn, that they became Eorzea's "fix-it" team whenever there was some stupid problem the cities couldn't fix; as the Path they were more secretive and I suspect a significant portion of the trouble the player went through in ARR would never have happened if they had stayed secretive...
Missing Uematsu 1.0 music (specifically, The Seventh Gate, the original Uematsu Piece of Mind (not the Soken-composed Gnath theme with the same title in Heavensward), The Forest's Pulse, and the old 1.0 Victory Fanfare)
The missing parts of Eorzea that had zone maps in 1.0 but did not return in ARR (specifically, the West Shroud portion of the Black Shroud, Coerthas Central Lowlands and Coerthas Eastern Lowlands for Coerthas (although the later is now part of a PvP instance, I'd still like to see it return as a proper area), and the entire missing southern portion of Mor Dhona towards Silvertear Falls, plus the underground 'public dungeons' like the Mun'tay Cellars and Nanawa Mines)
The old levequest 'factions'

[ReynTime]

Either bring these back or improved versions of them:
-Hamlet Defences (including the chocobo caravans tied to hamlets)
-Open world dungeons (for a different purpose than that of duties)
-Battlecraft levequests with difficulty tailored for parties
-Battlecraft leves as a reliable optional but limited source of exp using the overworld instead of being irrelevant compared to duty finder spam
-Momentum animations without locking characters in place (those already remained in cutscenes since 2.0) outside cutscenes
-Faction system/lore
-Bonus Chest system (preferably without the speedrun chest) for new optional dungeons if they ever add moderately more challenging 4/8 man dungeons not copy paste of the ShB dungeon formula
-Breakable monster parts that aren't just a few in the game for spectacle sake
-Battlecraft levequests having unique music per nation instead of the generic theme all of them have now
-All the missing tracks Uematsu made, specially the likes of Desert Moon Defied, Bathed in Woodsin and Forest's Pulse
-Battle Regimens (though they'd have to be improved a lot)
-Adventurer's Guild task boards

And I would add making every nation have 1 map combining city and fields, but since Kholusia supposedly capped their memory limit for a map and its not even that big... It would have been impossible for them.

[dreamfisher]

... the Path of the Twelve itself - I dislike what it became as the Scions of the Seventh Dawn, that they became Eorzea's "fix-it" team whenever there was some stupid problem the cities couldn't fix; as the Path they were more secretive and I suspect a significant portion of the trouble the player went through in ARR would never have happened if they had stayed secretive. ...

I agree so much with this. I love the way the mysteries underlying the Echo were presented in 1.0, too. I'll never forget the line

Not one is willing to speak of what he saw at the end of the path. It's as if they all bore witness to a truth too terrible... or too shameful to be told.

Or Papalymo's

The Echo has sounded... And nothing can stop it now.

For all its flaws in story-telling, 1.0 had an atmosphere teeming with dark intrigue. Though it's a bit of a vague point, I'd love to see more of this.

[Packetdancer]

I feel like the issue there is that the Scions aren’t the Path of the Twelve. But they aren’t the Circle of Knowing, either.

But the resulting merged group seems to be trying to still hold to the individual charters of its two component groups, despite the fact that one (the Circle’s “Sharlayan renegades throwing aside tradition and coming to try to aid the Eorzean city-states“) doesn’t really fit well with the other (the Path’s “we are the secretive, hidden organization that studies the Echo and tries to find ways to use it for the greater good“).

It made a lot of sense that the secretive Echo-powered group would aid the public “we must organize and rally together to solve problems” group. It makes somewhat less sense for them to operate as the same group from a practical standpoint, even if it makes NARRATIVE sense that Louisoix would tell the other Archons of the Circle of Knowing to go throw in with Minfilia if he didn’t make it back from Carteneau.