More interaction between creatures - it aggrevate me that certain hostiles attack you together, but not each other. Like why do Chocobos and marlboros attack you together?? (don't think it happen anywhere, but you get the point)
Roaming heards of animals.
We got fish tanks, would be cool if you could see those fishes in the water where you find them.
DoL NPCs Roaming around gathering stuff.
Ya, reacting to your character, it would be nice if some animals showed curiosity also, like cats/dogs coming up to you, and you can Pet them or such. Emoting NPCs, and they emote back. Kids running around with minions.
Seasons.

Seasonal content, like Feast, but with normal zones. I was thinking past few days it would have been cool when ARR started, that getting past Garleans fortresses should have been more of a combat oriented version of Ishgard restoration, but instead you help Craft supplies and items for raiding. So higher level content is gated for everyone, until server clear that Gate, so you can progress in story. And in current story, it would have been cool if whole server was more engaged in dealing with what's going on, instead you just pop into a 1man instance, and deal with the whole problem by yourself. Maybe a bit like Eureka/Bozja where you farm Fates, but with everywhere the towers popped up, and Lunar Primals showing up as S ranks.

Roaming Odin used to be a thing, would have been nice with more of that. Like let's say Garuda appear, and start rampaging through Black Shroud, attacking settlements and Gridania, and server need to stop her before she destroy everything - then you get seasonal event to repair the damage.
This would be nice with above content too, with Garleans raiding resistance camps, and players help fight them back.
To go a few steps further into wishful thinking, they could keep changing the Geography of the maps, to keep it interesting.
Just an excuse to do another calamity and remake the game again in Unreal Engine 5, to utilize Nanite and megascans. Well also the current one is really limiting.

Add new zones to old states, like lv 50-90 in Shroud/Thanalan/Limsa.

There's so much stuff they "could" do ^^ with infinite budget, and 0 development time.