Okay so lock down, despite still being working, has left me with a lot of downtime - as I am sure many of you are also in the same boat.
And to be honest, I came across a couple of rather interesting books.
This is kind of tied into Pundit's Abyss thread, in that Champions Mystic World would be a great for this, in that it uses the same ideas from Gurps Cabal, just in more detail. Such that it has on the tree universes that run exclusively on magic (Exalted crossover anyone?), or to those dimensions that have rulers. You could then have Briar and Atziluth as the province of the Primordials i guess.
Although a better source book is from the Age system - this is the absolutely superlative Threefold. It has three types of world lines - Netherworlds (Demon dimensions), Otherworlds (magic can be freely used), and Primes (where technology is used, but magic can be used by some / few). The cosmology is interesting, in that it can easily incorporate the Lords of Olympus stuff, and allow for Amberites etc.
It has some multiverse travelling factions such as the Divine Empire, Sodality and Peridexion (kind of UFP across planes), and the Nighthost. I absolutely plan on using this myself at some point, veeeery soon.
Let me know your thoughts.