Although the novel series was cancelled after the third book, creator Lisa Rollins had planned at least three more books. This "Second Trilogy" had a detailed story arc planned, and it was going to feature both characters from the novel series as well as the original TV Show.
Although the details of this trilogy is blurried, with the details stated by the author, as well as the apocryphal copies of the 4th and 6th books, we knows these plot points were going to feature.
After a couple adventures with them, it would be revealed Red Mary had returned. This would cause Auburn to go on a quest to discover how to get rid of her for good. She would discover from the Ancient One(s) a spell that could only be cast once every 130 years, and required each of the ten magic alignments. It is the only way to destroy an Abyssian. Auburn would then return to the crew, and tell them what she learned. First, they decide to find someone aligned with Death Magic...the Skin-Taker.
After learning Auburn is Lillian's daughter, the Skin-Taker decides to help destroy Red Mary to avenge Lillian. He makes a trip to the Abyssal Kingdom, where he recruits his sister Thera - who is aligned with earth magic - to help. It is also suggested he would either find Horace Horrible or use necromancy to revive him from the dead.
There would be several chapters depicting finding beings aligned with magic, including Talapio's rediscovery.
Eventually Red Mary would be destroyed.