Depends, i mean, once you get past that 10th level, it's up to the DM To structure how surreal they want to present the other plains as. History's full of heroes invading hell for one reason or another. you can have the demons that just populate the place, the 10th-11th level character's equivalent of kobolds and goblins, and then DEMONS running the show, ruling their plane of existence with a scaly fist, etc.

There are also other places to visit besides hell, where some of the more reality-defying elements can be thrown about with impunity. floating castles and islands, forests of crystal trees, etc.

They can all be treated as interesting places to visit, even if one wouldn't want to live there. I'm looking forward to hearing more about what you've got lined up. Tonight i'm going to go through some prep for this weekend.