I hope this hasn’t been posted elsewhere already, so ICYMI: A blog has released what they say is a Ultima Underworld 3 design document dated 1997 (free download). They provide no evidence, but it looks authentic.
If I am getting it right, the story would have taken place some time between Ultima 5 and Ultima 7 (that is before Underworld 2). It’s about how the Guardian came into existence.
It sounds pretty good, maybe there is a chance it will still be realized in case the Ultima brand goes back to OS at some point in time.