The issue with FFXIV IMO is it's design is very linear like single player story driven games tend to be. Sure there's side quests and small detours here and there. However for all the space given in it's overworld zones there's hardly anything to keep players around longer than the few levels they spend doing the quests in the area. The other issue I feel is the overly excessive removal of RPG elements and mechanics from the game.

Yes I agree the next FF MMO should take a fully open world design as it's base meaning it can be played numerous different ways from exploration, sandboxing, following the story, questing for gear/skills, etc. Return the mechanical depth to the game to at least match FFXI where things like damage types, elements, etc matter and jobs aren't pigeonholed into having only 1 way to be played. Lay out all the options and let players choose what they want to go with based on how they want to play the job so we have numerous different paths via horizontal progression and ways to tailor the job for the specific content we want to do. Take some pages from other open world games to create repeatable content that will give players reasons to revisit areas regularly. I'd say even follow Xenoblade's example of having wide level ranges of enemies present in the same areas.

I'd even say go back to the Basic/Advanced Job classifications we had previously where it wasn't a matter of Advanced jobs being better or more powerful but simply having a higher skill curve for playing it as the Basic jobs available immediately generally focused on 1 thing while the advanced jobs combined aspects of 2 or more basic jobs.