I think that Pandemonium, Carceri, and Gehenna could all be really cool hell dimensions. Though admittedly Pandemonium has been made kind of redundant with the Underdark. Same general idea, except with demons instead of tentacles.
Carceri is a prison world for the damned without having lots of big deadly demons and devils swarming everywhere to tear visiting adventurers apart the moment they arrive.
And Gehenna is just visually pure metal! Who wouldn't want to adventure in a world that is all volcano under a permanently black sky?
I have an idea for a mini-campaign in which the villain has an artifact from Carceri, which can exchange a soul from the Material Plane for a random denizen of Carceri. The artifact can also reverse this exchange, sending the Carcerian back... but if the Carcerian dies on the Material Plane, the soul is trapped in Carceri forever.
The villain is using the artifact to turn isolated villages into a motley army of fiends, monsters, and the damned. This army obeys the villain because he has the power to send them back at any time; however, they are absolutely treacherous, and what they really want is to see the artifact destroyed so they can't be sent back. (Mind you, they will still return when they die; the artifact is a malevolent trap, not an escape hatch.)
So, the PCs will need to discover what the artifact actually does, capture it intact, and use it to send the army back before destroying it. Unless, of course, they stop to think about the Carcerians they killed during the course of the adventure, and decide they have a duty to retrieve those lost souls. There are two ways to accomplish that: First, the artifact has the power to swap one soul for another, so you can retrieve the lost souls by throwing other people into Carceri in their place... if there's anybody you can convince yourself deserves that fate. Second, you can bring back the lost souls connected to all Carcerians you killed, if you're willing to throw
yourself in. Doing so will begin a new story arc where the PCs have to bust out of the prison plane.
(If the players don't stop to think about the slain Carcerians, or decide it isn't their responsibility, none of that last part will come into play. It's not something I intend to push on them.)