The notion that creation is subgradient, that in short everything becomes smaller and more numerous over time. Coupled with the notion that the Mundus is a tool for ascendance, an effort to reverse this fundamental constant of the Aurbis.
I've become increasingly obsessed with the remnants of Yokuda, the Eltheric ocean, and Thras as of late. I would like to see them fleshed out and get to explore them in a DLC, or more likely, an adventure game.
The Imperial civilization and empire even though there isn't much of an empire now though I wouldn't know too much since I haven't played Skyrim yet but they're my favorite race and their Aedric gods. Also the Tribunal demi gods and the Temple was most interesting and I liked Vivec or else called Vehk the most since Vivec seemed so slick to me.