The Hand of Darth Sidious: A Star Wars Theory by Yours Truly

What if the Palpatine we saw in Star Wars Episode IX: The Rise of Skywalker was not the "real" Palpatine at all?  What if he was some kind of replica without an iota of Palpatine/Sidious's spirit whatsoever?

There is precedent for such a notion.

In Timothy Zahn's two-part "Hand of Thrawn" novel duology from the late Nineties, the galaxy hears whispers about the return of Grand Admiral Thrawn.  Thrawn in the still-venerable expanded universe, you may recall, was killed toward the end of The Last Command.  The galaxy thought he was dead.  But he prophesied that he would return in ten years' time if he ever was thought to have died.

Long story short, it turns out that at Thrawn's hidden fortress (see what I did there?) in the Unknown Regions, the grand admiral had prepared a clone of himself.  It had been in stasis all that time.  Just waiting to be "brought online" by his faithful Chiss followers.  It would not have had any of Thrawn's soul, but it definitely would be Thrawn after a fashion.

What if Emperor Palpatine did such a thing for himself?

I can easily imagine that Palpatine went to Exegol and had himself cloned, and had it put in storage.  It would be waiting, for the "unlikely possibility" that Palpatine would die.  Who knows how but perhaps the clone was given the original's memories and abilities (it seems to have been the case for Joruus C'Baoth in the expanded universe), ready to be taken out of its vat and brought online.  The Emperor would be reborn.  But as Palpatine says in a deleted scene we know exists, he would be "more than a clone, less than a man."

So Palpatine/Emperor/Darth Sidious creates the clone and when his Sith Eternal followers get the news that he died and the Empire is finished, they activate the clone.  Just as Thrawn's people were about to do with his own clone.  And THAT is who we see in The Rise of Skywalker: a replica of Palpatine, created as a failsafe in case the real one was killed.

Such a notion preserves the idea of Anakin Skywalker being the chosen one who brings balance to the Force.  While at the same time restoring Palpatine as the overarching mastermind behind the dark events throughout the Star Wars saga.

Just a theory, I confess.  But it's one as good as any other.