I got another interesting answer to my quora.com question, here.
I recently asked a similar question on quora.com. Most of the answers weren’t very interesting to me, but one was. It’s here.
Still working on the Underworld. I’ve added cave-ins, jammed doors, and doors that ask you riddles. Next up, secret doors.
I just finished another section of My Name Is John Carter, so I should have more time to work on this for a while [edit: and just after I wrote that, I got a big editing job].