Honestly, I would like to see him go back to Exile, dust off some of the elements in that game and alongside Avernum, put the story together in such a way that the entire series comes out as a single game. At the very least, have 1-3 done in this manner.
Have an element in there so that you can choose to have particular characters stop adventuring and 'settle down' (not just delete the character) to make room for new adventurers and give the 'retirees' the ability to affect the story in some way, the opportunity to pick up the 'retirees' children who are pre-made characters but have additional strengths/weaknesses, or give the adventuring party access to gear that otherwise doesn't exist.
Not positive about the timeline. But, if it's spaced far enough apart, then by the time the third part of the game starts the first adventurers will have died of old age. If the player didn't make any attempt to take advantage of the generational stuff, they could suddenly find themselves controlling a brand new party in the pits of a destroyed Avernum. Should they have elected to not fulfill any of the game winning quests for those parts of the game. Where the Vhanati are running around killing humans because of the actions of the Empire in stealing Crystal Souls, the demon of the pit is still alive and ruling most of Avernum, and what little strength Avernum had was crushed by the Empire. Leaving the player with the knowledge of the location of all his awesome gear and a very difficult, as well as unlikely, path of getting a hold of it all. Because he's back to level 1.
I bring this up, because I see the choices of what the player is able to do to be steadily dwindling . Potion Making is gone and the spell list is a joke of it's former length and breadth. Finding Arcane Scrolls, means nothing but money and xp rewards for successful quests, when I feel that my wizard/mage should be able to use them to advance his/her abilities in his/her craft.
Then there's the whole Erika situation. Maybe I want my wizard/Mage to work out a way to break the curse she is under?
I guess what I'm saying, is that I'd like to see 1-3 combined, and from that, sandbox elements introduced where the story and available resources can massively change. Instead of having 'auto-die' or even 'this will happen because the story-line I'm writing demands it' type events force certain things to happen. The player can do things like fight off the Empire Army at the front lines, assuming that he/she has the resources and levels to pull it off.