ferratus said:
I don't like that the Nine Hells are an astral domain. If the devils are going to be the agents responsible for humanity's damnation from now on, they should be less on the periphery. The best location for devils would be entombed beneath Shadowfell. Since they are already hungering for human souls to increase their power, and undead are considered to be halfway to absolute damnation anyway, what better place for them?
That's not too unlike how I've done it. My pantheon is a bunch of very transparently slightly renamed demon lords and archdevils. Bel is the god of war, for example. Dagon the god of the Sea. Imix the god of the sun. Urkas the god of death. Yinigu the god of nature. Etc.
My cosmology and cosmological history is that at some point in the past, these archfiends won some major battle "beyond the fields we know" and the entire world and it's "planar neighborhood" is under "lockdown."
I had a coterminous plane that was not unlike the Fugue Plane combined with the Plane of Shadow, ergo, very similar to the Shadowfell, and the only destinations you can reach from there are various Hell Courts. Basically, the fiefdom of the individual archfiends and their courts, so the equivalent of a single layer of Hell or of the Abyss.
I did away with a lot of the fluff distinctions between fiends; demons, devils, yugoloths, demodands, heck, even efreet and slaad are all considered fiends under my cosmology and may freely intermingle. Any given court can have any mix and match of fiends that the Fiendlord happens to like around him. In the Plane of Shadow, you can find tons of undead; people trying to cheat death and linger. You can find fiends patrolling the area, looking for anyone they can bargain with to come to their hells for good and also, therefore, cheat death by becoming a larval fiend and possibly growing up to be a major player in the Courts of Hell. They also tend to hunt those who try to escape from the Hells, or simply those who don't belong or don't look like they can take care of themselves. It's a pretty wild and wooly frontier kinda place.
So, in many ways, not unlike your idea at all. Not unlike the direction 4e seems to be going either, although I'm going considerably farther than they are, especially in terms of lumping fiendish types together, at least in terms of fluff.