Personally I really don't like the idea of the Sith coming back at all.
Spoiler!
First of all, if they bring Palpatine back, I'll probably walk out of the theatre. It would basically reek of fanboy service and panic. If they bring him back in a holocron that would make sense, if through the Holocron he instructed Kylo Ren, that would work. But please as much as I love Ian don't bring Palpatine back to life just to kill him off again. It would just be stupid.
I can see that, the climatic scene in the movie, Rey and Kylo are facing off.
Second of all, bringing back the Sith or Palpatine spits in the ending of the Anakin sacrifice and all of Luke's life and quest.
The Sith died out when Palpatine went down the shaft and Vader died. Thousands of years of the Bannite Sith line and all of the knowledge and power acquired was gone, there was no successor to the line. Snoke wasn't a Sith, he was just a dark side user with a streak of stupidity. Ren isn't a Sith.
This just feels like panic after the lashback from the last movie.