I might have already been spoiled on the design of the final boss of ER, but I really loved the theme, given that it's based on the theorized interpretation of what the universe looks like zoomed out the furthest it can possibly be. So far out until all of our galaxies, nebulas, stars, planets, etc. end up looking like one big tree with various branches. It all felt very fitting, especially with the connections to space and whatnot.

https://files.catbox.moe/86ey2a.png

The closest I could find to an image of this interpretation is from Tetris Effect.