I’m sure making Sifo-Dyas a Sith apprentice was one of George’s ideas at one point, but both the original EU and Clone Wars season 6 (which was technically released under Disney but written with George’s direct involvement) officially established him as a well-intentioned Jedi whose drastic actions were discovered and exploited by Sidious, who eventually ordered Dooku to have him killed.
Definitely an intriguing idea for a fanedit linking the trilogies, though.