I don't think it's wise to change an art direction mid game, but if we were just talking in general- I kind of hope they can make their next one look more 'art-like' akin to the concept art they use themselves. We've seen recently a lot of anime like visuals done very well in other games, and I'm not saying that exactly, but I think it could be interesting to have a sort of style similar in concept art they've shared (more stylized art style- which can also help it age a lot easier, a nice feature given MMOs tend to last a while). Also helps further reason it's a different game rather than 'new game higher resolution but that's it' sort of deal.
Definitely how memory is handled in this game is the main engine issue I thought of first when reading the thread though (hence I referenced glamour, our housing ward limits also relate to this where some servers have too many houses and others have not nearly enough). Collision / movement could be another nice one, though that depending could be an insane task (depending on how the system was done.. could be unreasonable).
For graphics, of course higher textures and all that would be nice, but generally I think most textures are fine it's just specific ones that are very egregious and if they could fix those it would automatically be better (some of which players have made mods for to fix lol). Personally I'd love to see improvements to the light, water, weather, and flora engines. They're not awful, game isn't ugly, but I can imagine them (in my mind sort of thing) as being fairly meaningful changes to my moment to moment wandering. Such changes also potentially not requiring the update of thousands of textures but more of a system that might be more moldable than memory (so they're more 'do-able' lol).
For terrain I'd love to see them be able to use better LOD assets- like thinking of that very pretty zone in EW that is Zeal (chrono trigger) like... I imagine it would look even better if they had some better distant objects. My GPU should be able to load the entire zone, but I still have pop in and stuff for those grand vista views that I feel a better engine could greatly improve. I'm sure there are other cool tools and expanded features that could also be used to help, not that I'm knocking the team- I quite like FFXIV's environments but to me in general a /major/ reason why I like FF games is the environments and so investing in the power and use of that power for environmental design is always a boon to me. Like it was absolutely great they were experimenting with verticality for the Khoulsia zone, but it was a little sad to hear they tried to get the last bit of the zone 'actually' in the water but the engine was like NO, STOP!!! lol (still cool they were pushing the engine though). I'd prefer when the concept art team and the environment department get together they can just do whatever fills their dreams (which is much easier said than done of course).