Adventures of Blackthorn was planned to show the fate of Blackthorn after being banished by Lord British at the end of Ultima 5. The project was abandoned in mid-February 2004, for reasons that I only brought to light at the end of June 2004.
Worlds of Ultima: Lost Sosaria is best described by Aiera:
Pursuing Fellowship agents deep into the bowels of Britannia and the ruins of the Great Stygian Abyss, the companion Sentri leads his party of former friends and companions of the Avatar into an unknown part of the world: Lost Sosaria, two of the continents of Old Sosaria left missing after the death of the Wizard Mondain.
Lost Sosaria is not set in Britannia, but is about the discovery of new places, as is the tradition of the Worlds of Ultima games.
Chronicles of Batlin was planned to allow you to play Batlin's role, uncovering much of what he did before he was thwarted by the Avatar. However, their host took down their website, and they haven't yet reappeared. Even their cjb.net redirection has been removed.
Ultima - The New King originally aimed at producing a sequel to Ultima 9: Ascension. After officially merging with the Redemption team on 18th September 2004, it is, instead, going to be a sequel to Redemption.