The Nine Hells in my game is very similar to the Nine Hells as they have changed (slightly) throughout the editions, from when I started DMing back in 1983. Moloch was replaced by Malagard was replaced by Glasya. Fierna showed up to spend some "quality" time with her dear old dad Belial. Bel took over from Tiamat (or Zariel). Apparently, Zariel's back now. So, that's cool by me as well.
There are a couple of slight differences in my treatment of the Hells, though.
1) The Nine Hells have always been on the Great Wheel in my campaign, and they have always been visualized as an inconceivably huge "pit" of 9 infinite layers. I openly laughed at 4e's treatment of the Nine Hells as a bounded world with "caverns" that were large, but measurable. Most countries in my campaign world would have been bigger than layers of the 4e Nine Hells, which are supposed to be the Hells of ALL the campaign worlds of the entire D&D multiverse. That's just ridiculous. Every layer of Hell is infinite, as are all the other Outer Planes. They're too big for the human mind to even comprehend. Anyone who tries to provide a complete map of Hell, or even put a measurement to it, just isn't paying attention to our own Real Universe. Something a couple hundred miles wide is insignificant compared to the scale of only our own Earth, and absolutely laughable compared to (say) a single galaxy.
2) The archdevils are gods. That's all there is to it. Not every campaign setting has Paladine (Dragonlance) or Mystra (Realms) or Wee Jas (Greyhawk) or Aoskar (Planescape) or Usamigaras (Mystara). But virtually every campaign setting, even the homegrown ones, have Dispater and Asmodeus. They rule entire layers. Not even Thor or Apollo or Osiris rules an entire layer. If the PCs ever get to fight one of these guys, AT BEST they're taking on an avatar. No PC in one of my campaigns will EVER get to permanently destroy one of the big guys, or take their realm. All power to Dianthus the 30th level pixie vampire, but she's going to be a Tiny-sized stain on the wall of Dispater's Iron Tower in one of my campaigns.
3) Bel and Dispater, in particularly, are openly worshipped in my campaign world... but not under those names. Bel is known as Tienju Ko ("Lord of Gifts"), and is the LE God of Pride and Ambition. His worshipers see him as a benevolent god who rewards political and business skills, with the strongest and smartest rising to the top. They're a lawful group who support a predictable, peaceful and organized society... but are also completely self-interested capitalists who believe altruism is a fool's game. Any resemblance to real world philosophies is purely unintentional. Dispater is worshiped as Sakodo ("The Wise One"), LE God of Wisdom and Prophecy. Again, not considered an evil god - but his teachings certainly promote an orderly and self-interested way of life.
3a) As a general comment, virtually none of the evil gods in my campaign are actually considered evil by their followers. It has always bugged me that so many gods in so many D&D worlds are eeeeeevil. There's very little motivation for worshiping someone like Cyric or Tharizdun or Bane or Hextor or Tiamat. They don't treat their servants very well, their afterlife is known to be an unpleasant place, people tend to hate you and everything you stand for, etc. Any evil god with an intelligence above 8 should be portraying themselves as a decent deity that rewards their servants well, runs a paradise in the afterlife, will improve your standing in society and reduce your chance of getting your head hacked off. Their faith can be corruptive, can require unreasonable sacrifices, and can lead to some form of damnation... but the path to get there should be pleasant, seductive and non-confrontational. More sparkling churches with handsome priests and beautiful temple dancers, discounted healing potions, and charitable outreach programs in the community. Less spiky onyx temples with piles of skulls, diseased and malformed social misfit priests, who demand you offer up your firstborn and mate with a demon.
4) And, following on from the above, Bel is still around in my campaign world. I'm okay if Zariel has kicked him off the throne of Avernus, but he's still a powerful (deposed) archdevil/demigod in my campaign setting. Same as Moloch or Geryon. They're still kicking around somewhere, mad as heck and wanting to get back on top.
