I would like to explore the empire again, and see some expansions to it. I would also like to see what happened to those crazy ol troglodytes.
For Avernum 6, I believe Jeff already wrote a really good potential story line: the overcrowding of Avernum. What I could see initially is squabbling and infighting in Avernum. As resources become more and more scarce, groups begin to get violent again.
Then, in the middle of it all a great catastrophy occurs: the caves themselves, turn on all the avernites. Seeing as the caves are magical, I thought now would be a good time to introduce what makes them magical. I'll call this driving force the 'originals'. Anyways, the originals find that their avernum is being torn up, and decide to teach everyone in avernum a lesson.
I thought it would be interesting to introduce a new enemy: fungus! There is fungus everywhere, which would make this type of enemy really unpredictable and frightening for the cave dwellers. Think how you would feel if the food you tried to grow suddenly became smart and tried to kill you. Anyways, the originals use their magic to warp various fungal growths in avernum. They use the fungal growths to attack avernites etc etc
Using undead would make sense as well etc etc
I thought the above might be interesting, and if anyone thinks so I can extend it.