Seeing this done for one of the factions from Planescape made me immediately think of what other groups could be given greater spotlight in similar materials, and the various deities of the D&D multiverse was the first to come to mind.
I can imagine a supplement going into greater detail on the various versions of a god in different worlds they appeared in (maybe even including a First World interpretation) with various lore about their ideals, realms, clergy, and dogma. They could even include some monster stat blocks for a high ranking follower or champion or unique monsters and extraplanar creatures associated to them. Also maybe include a set of piety rewards, unique blessings and boons, maybe even some unique spells or artifacts from older editions.
It would be a cool way to get more deity material while allowing those not interesting in them to ignore the content.