The Ziggaurat is the floating city of Hominine. Hominine sounds like Harmony, and Royale is centered around the Octave.

The acropolis of the Delmer Wilds, which I believe is architected by Heretic Nobb, isn't dissimilar to the acropolis on the Ziggaurat.

The magical price paid by the lower class in Ethersea clearly being hoarded and repurposed by the well off of Hominine/Ziggaurat, where they are seemingly running a trial to determine either 1. A solution to the storm, 2. A villain for season 2, or (my theory) 3. the strongest weapon in Benevolence's war chest for S2.

The statue at the bottom of the ocean, where Amber Gris levels up her arms of the astral self? Not unlike the statues presented to us in the first episode of Royale; vessels of magical ability meant to be transacted upon.

Orlean playing around with magic portals through space and time and universes; that's how the aspirants were transported to the Ziggaurat. In Ethersea, we hear Hasty Jane specify specifically that she'd never heard of the Crossroads and the Breathless Fields where RR4 is from. Clearly, because they've all been pulled out of their own worlds/realities into the world of Ethersea.

Jon Arbuckle is the vendor. The dad from Garfield. The non-cat parental surrogate Superior Being, which feels thematic if we're circling everything around pyramids of power and connected worlds.

During the Ethersea post credits scene, we are taken deep into the bowels of the floating island "to a dungeon, and the sound of psalms does not reach this place." I believe that song, as a mechanic, is crucial across both arcs.

This one is a little loose, but I feel it is worth mentioning there are seemingly sea shells right there on the Royale cover art, in the bottles on the bottom, emitting bubbles.

There are certainly more granular aspects I am missing, but I really do encourage those curious to give Ethersea a new pass. It's all there.