What difference did the Twelve tangibly make?

[Wix]

I think Myths of the Realm really took away the unknown from the game,

I felt unsatisfied with the development as well. I thought, given time, I'd come to like the ending more, but mostly it's just left me feeling kind of bitter? I liked it, story-wise, but not as a world-building development.

at least in the terms of Eorzea itself. Im sure there is stuff here and there, but for the most part it seems like Eorzea is finished content. I don't really know what they could dive into that hasnt already happened before, or that they don't just make up after the fact with no connections previously.

There's unfinished stuff with Duskwights and Gelmorra, but... they've been pretty deliberately moving away from them since ARR. They even killed off-and-replaced or retconned some story-prominent Duskwight characters into Wildwoods (Estinien being the most prominent example).

My tinfoil hat belief is that it's because they don't know what to write for Gridania and struggle to write the Elementals without making them come across as villains, so everything Shroud-related is just in eternal limbo. Incidentally, Moon Cats similarly get 'trapped' by this.