As I've mentioned before, I've realized that what makes sense as a followup to the Basic Psionics Handbook is some sort of "Basic Planes Manual." It actually started as an appendix to the psionics book, but I realized the space was limited, and with all I wanted to include, it warranted its own book.
A while back, I also mentioned (in a G+ comment, I think) how I really liked how Tom Moldvay handled his hierarchies of planar creatures (mostly elemental planar creatures) in Lords of Creation, and that I was going to borrow that model for the planes book.
One of the things I've been working on lately is a hierarchy for the Abyss. I know that demons are not ones to submit to a hierarchy, but it's really more to organize the strength of the demons (from weakest to strongest) so that DMs can (at-a-glance) know which demons would make suitable foes for their players (based on level, monster HD, special abilities, etc.). With that in mind, I set about trying to define the hierarchy. I took some cues both from LL and from the AD&D MM, as well as expanded the demon lords a bit to include some classic Sumerian names. Here's where I ended up.
You'll see that I don't use the same strict HD limitations for order as the LL AEC. Instead, I use a more discriminating line based on whether or not the demon has psionics. (That's not an available cutoff point for the AEC, since there are no psionics there). It also makes something like the marilith more formidable in my system than in LL. Thus MY Mason-Dixon line for standard vs. higher order demons.
As a support to the above, I started to work out my own cosmology around the different layers of the Abyss (again, with some nods to the D&D history of the Abyss, but while trying to create unique and/or OGL-able versions of the names) to "place" the demons and demon lords with their own realms. And here's what that looks like so far.
This is all work in progress, but I'm to a point where I'm done fiddling with this for now, can slide it to the back burner, and get back to the more pressing/ongoing things that SHOULD be occupying my attention.