No one knows what Star Wars Episode IX will be about, at least not yet. The only ones who know are the cast and crew, so we're going to have to wait and see for a trailer to get an idea on how this trilogy will end. We know that a number of actors will be returning from the last film, like Mark Hamill as Luke Skywalker and (surprisingly) Carrie Fisher as Leia Organa in a scene that was filmed prior to her death.
One actor fans are hoping to see return is Andy Serkis as Snoke, who many felt had an anti-climactic death. Serkis recently had to cancel an appearance in the Keystone Comic Convention, with many theorizing that he is going to appear briefly as Snoke in a scene or two. Considering how Episode IX is currently filming, there's actually some basis on this theory, even though it could be dumbfounded.
That being said, celebrities cancel appearances in cons like this all the time. Sometimes it's because of schedules and other times it might be due to personal matters. Serkis has yet to release a statement but it will be interesting to see what he has to say.
Supreme Leader Snoke was one of the most intriguing parts of the new Star Wars trilogy, so it's easy to see why so many people were angry over his death in The Last Jedi. The fact that he was able to lead the New Order after Return of the Jedi intrigued many since we all felt that the Resistance had finally won. His return, whether it be a flashback or some sort of revival, would be of great interest to the fanbase who want to know more about this mysterious Sith figure.
Star Wars Episode IX is slated for a December 20, 2019 release.