I bet on PS5 just backward compatible with PS4 and you can play same XIV you have now.

If and when PS4 support will be dropped, we'll likely to see other kind of improvements, as happened when PS3 dropped and max number of concurrent objects incrased to PS4 specs.
But about overall graphics goodies improvements, they have already been ruled out because the cost is simply too much to ever think at it, too many assets are to be redone.

I guess we'll see better overall graphics on next FF MMO. Let's kiss clipping goodbye? xD