Fanfiction can change the tragic ending by simply changing one key decision. For example, if Anakin decides not to trust Palpatine's lies about saving Padmé from death. This one change could lead to a completely different outcome where Anakin remains a Jedi and the Sith's plot is foiled.
One possible alternate ending could be that Anakin Skywalker resists the dark side at the last moment. Instead of becoming Darth Vader, he uses his love for Padmé to find the strength within him to reject Palpatine's temptations. This would change the entire future of the Star Wars galaxy, as there would be no Vader to enforce the Emperor's will, and the Jedi Order might be able to be rebuilt more quickly.
The novel of Revenge of the Sith is canon. It aligns with the established Star Wars canon framework and adds depth and detail to the events depicted in the movie of the same name.
Anakin Skywalker and Obi - Wan Kenobi are main characters. Anakin's fall to the dark side is a major part of the story, and Obi - Wan is his former mentor.