Yeah, I think one of the most common hopes I see people have is that Anakin’s Force ghost will show up. I know some people wouldn’t want that, but I think if Anakin, as a Force ghost, somehow took part in purging the last remnants of Sidious from the galaxy, it would help keep that prophecy/story element from George in tact.
It would actually make a lot of sense. If Sidious was in a corporeal form, and can’t physically be destroyed, it would present a great challenge for Rey, whose physicality is one of her strengths. If she was shown to struggle to tap into her spiritual side throughout the whole movie, and then at the climax she completely let’s go of her own fears and puts all of her faith in the Force, maybe that would be the way she has to defeat Sidious.
I’m going to ramble on somewhat of a theory regarding Force ghosts. In ESB, Obi-Wan tells Luke that he can’t interfere in his confrontation with the Vader, but maybe that is because someone on the spiritual plane can’t physically harm with someone on the physical plane, and vice versa.
I also think it is possible that living Force users have to act as conduits for Force ghosts to manifest themselves, so Force ghosts can’t just appear on their own alone. So imagine Rey being unable destroy the spirit of Palpatine on her own, and in her most hopeless moment, she finally let’s the Force flow through her completely, and suddenly the Force ghosts of Luke, Leia, Yoda, Obi-Wan, Qui-Gon (and Ahsoka?) appear and join forces to try and purge Palpatine’s spirit. Then finally, Anakin appears and delivers the final blow, destroying the last remnants of the last Sith Lord from galaxy.
I know I’m just setting myself up for disappointment, but it could be a good way to tie into Rey’s characterization, make the Force ghost ability that Qui-Gon apparently discovered in Episode I suddenly very important to the overall narrative in Episode IX, and also maintain Anakin’s role as Chosen One.