How about a jump back into history????
You can't play the discovery of Avernum, that was Empirical forces, and by history, they all died...
But, You should be able to play "The First Inhabitants"...
You would be sent down shortly after the Triad lost their power struggle...
Avernum would mostly be unexplored and dark.
The Portal fort/castle would only be a fortified camp site.
And shortly after your arrival, it gets attacked again.
As a new arrival, you could be used for camp defense, assigned to a post on the wall, with a bow.
Later, you would become a scout, assigned to scout the area, and locate the native camps, towns, and cities.
But, you are to report back their location, not attack...
Later, you would ranked as raider, you will be sent out, and for larger camp sites, with other troops, to capture/clean-out/kill the enemy camps...
Included side adventures would include:
Planting cave trees to make forests... you will be given a map to say where to plant these.
Launching cave fungus up to the ceiling for later cave lighting, map directed...
maybe even scout for other city sites and a location for the wizard tower...
And last, after the main cave demon is found, you will be part of the force that defeats him, and are present when the Triad traps him in the jar.
One side idea, you make your party of 4, just like before, and you are tried for "treason against the Emperor" and each of your friends are sent down, but, 1 at a time...
You are sent down second, a friend, (#2), was send down 2 or 3 days before, but you don't meet up with him until after the first battle. When he finds you, you are presented to the camp's leaders and told about how good the two of you would be a scouts. They will 'think' about it. And every 2 or 3 days another of your party is send down, When your party is filled, you get your first scouting mission.
(Yes, I have thought about this idea for a while)