While Star Wars: The Rise of Skywalker began to answer and definitely closed off certain aspects of The Skywalker Saga story, it also introduced a lot of elements from Legends and Lore, and left certain things seemingly ambiguous.
With that said, the marketing for this film was not shy about about the return of the series' biggest villain: the former Emperor, Supreme Chancellor of the Republic, and super manipulative Sith Lord, Darth Sidious.
But there are things mentioned or shown in the film that are curious:
1. While Sidious reveals he's Rey's grandfather, he also mentions that all of the Sith are with him (or that he is all of the Sith).
2. He is not fully formed and, at first, hangs on an apparatus.
3. He is found on the alleged Sith homeworld of Exegol (and there may have been statues of Darth Nilhilus present).
4. He mentions that he has "died before", and we may presume that a combination of dark side Force power and genetic manipulation have been involved, as there are containers full of Snokes!
5. There are not Sidious loyalists present, but rather "Sith" loyalists and a Sith fleet with Sith Troopers.
6. The opening crawl's first words are: "The Dead Speak!"
In addition, episode I’s title, The Phantom Menace, is meant to more specifically pertain to Darth Sidious, besides being able to also apply to the tragedy of Anakin Skywalker.
Also in the prequels, the Jedi comment that The Sith have returned. And although it's most likely that they are just referencing Darth Maul and Darth Sidious, it does suggest a possible awakening to the Sith generally, as opposed to just dealing with a master and his apprentice(s).
Thus, it almost seems that Sidious (or the Sidious in Star Wars: The Rise of Skywalker) is a byproduct of something beyond the previous Sidious, or that perhaps Sidious himself was always answering to someone else? It feels like there is a door behind a door with the way the character was so elusively presented in this film.
Is there any evidence from other upcoming projects or the executive producers that suggests that perhaps this Sidious isn’t exactly the same Darth Sidious as before?