There's a lot of speculation that Palpatine / Darth Sidius will be in the new Star Wars film after the death of Snoke.
In the Legends novels, Shiv was brought back via cloning methods.
And if they can bring back Darth Maul for the Solo movie, they can bring back the Emperor right?
I actually really doubt it as Star Wars needs new characters to give new life to the franchise. None of this he's coming back as a ghost, we need a new villain after Snoke and whatever Kylo Ren is becoming.
One even worse than Jar Jar...