The most lacking areas in this game that would be easiest to exponentially improve are in my opinion

Lack of Grand Company related -questlines- (Like, actual plot that is episodic based upon your rank. Not Beastman dailies persay, just a handful of story quests per rank much like the Postman content has been episodic, in the same manner based on the release of ranks or something akin to the Job/Class questlines)
and
Lack of NPC companions outside of the Chocobo who I find hard to develop any meaningful attachment to or desire to use. Easy fix would be to just allow us to party up with your retainers when soloing, I mean we gear them up for ventures and can see all their stats so it seems simple enough to implement. Alternatively add an adventuring fellow companion that we're allowed to customize ala FF XI or FF XIV's path companion. In the implementation of the latter they could even have their own questline much like in XI to boot, for example every "X" amount of companion levels you unlock a new quest, boom.

There are a ton of other things I could recommend but those two seem easy to implement working within our current constraints and do a lot to flesh out otherwise underutilized systems in game.