My final item for the Ultima Underworld 20th anniversary is a game story design document for the never released Underworld 3. This details an entire potential storyline for the game as of August 22nd 1997 and is available as a pdf from here.
The Ultima connection appears to have been dropped in favour of another fantasy world and the game is far less dungeon based than UW1 & 2. The document is only a brief outline and there is precious little about gameplay unfortunately. I find it hard to form an opinion of how Underworld 3 might have turned out based on this alone but the scale of some of the events was certainly ambitious for 3D engines of the time and the potential was there. It’s hard not to think that this would have been in direct competition with Ultima 9 which could be one reason why it never happened.
There is far more detail on Underworld 3 in the archives at the University of Texas. That would be something of a lengthy round trip for me and all I’ve seen of it is on this Facebook page.