From what we saw so far, good old Ian McDiarmid once gets the chance to play the Big Bad Villain Who Controlled Everything From The Background character necessary in every adventure movie since... the first James Bond movie I think? That's all well, I love him and his character played the same role in OT, so let's assume I'm right about he will be a central figure again. But we can all agree that Palpatine has absolutely NO place in the last movie (please keep reading! Let me rant about a bit before I get to the point). Having him survive this long completely messes up everything established in RotJ.
First and foremost, this makes the first six films completely pointless. If Sidious survived, then Anakin Skywalker isn't the Chosen One who brought balance into the Force, but a pathetic random guy who was fooled by him. His whole arc, the very reason of his existence would be negated! And no, begetting Luke and Leia who may or may not help Rey who is totally unrelated to Anakin is way too far-fetched to be a fulfillment of the prophecy. I can't imagine any way to salvage Anakin's arc and still keep Palpatine alive.
Second, it wouldn't make sense for Palpatine to go into hiding for decades. Even if he's seriously wounded, a message that he survived would have been devastating for the Rebel Alliance. "You can destroy my flagship, you can destroy my Death Star, you can kill my apprentice, but you will never defeat me!" Just imagine the impact... The rebels lost almost their whole fleet and the Emperor is still alive! Do you think that ANYONE would dare to join them now against an indestructible Emperor? And imagine the morale boost for the Imperials! Even the Battle of Endor would be seen as a tie at best. We know from the books and Battlefront II what a disarray the Empire was in after the death of the Emperor. With only a short text message CCd te every admiral would have spared all the turmoil.
Third, we know from the still canon sources that Palpatine actively used the battle meditation technique during the Clone Wars and the GCW as well. And his specialty was that he could use it from afar, manipulating battles lightyears away. Why would he stop supporting his troops if he survived? And the movie is directly contradicts to Battlefront II too. We can be fairly certain that the cession of his Force control due his death activated those droid messengers. And why would have Palpatine order the destruction of the known civilization if he was planning a return?
It just doesn't make sense for Palpatine to go into hiding if he indeed survived. Even if he's seriously wounded, he had a whole Empire to defend him, all he had to do is to return to Coruscant.
So why is he in the movie then? Is this another way of Disney to boost the nostalgia factor? They would be so dumb that they don't realize that many fans are sic of the re-re-re-re-reuse of the old characters? They really had no better idea than pull out a Sith from the hat? Couldn't they invent a new supervillain? Or they just wanted to troll everyone? Well my guess is that they really DIDN'T want to bring back Palpatine, because of the reasons I listed above. They did create their new supervillain, called Snoke. They even gave him his own Darth Vader, but instead of only one, they created a whole Order of Extraordinary
Gentlemen
Force Users, the Knights of Ren. I think that was JJ Abrams' original, but when Rian Johnson got the directorial seat for the second movie, it all fell apart. As the story says, good old Johnson threw away Abrams & Co' script arching through all three movies and created his own movie. The result is well-known. Ridiculous plotholes, pointless adventures, laughably dull conversations. And the most pathetic death of a supervillain in movie history. Oops... I don't know how this passed on Disney, but I'm pretty sure Johnson killed off the supervillain who originally was to be defeated only in the climax of the last movie. So JJ had no real choice. He had to find a new supervillain and having only one movie, he didn't have time to introduce a new character and make them menacing and significant enough, while he also had to wrap up the story of the main characters and decide the fate of the Galaxy as well, and order it all into the three arcs of classic Greek dramas. So he had to turn to already existing materiel, but there wasn't that many great villains left alive. Jabba died (quite spectacularly), Vader was redeemed, Thrawn was never properly introduced on big screen, Snoke was cut in half, Palpatine died.... OR DID HE? He fulfilled both criteria (being known to spare the necessary introduction and possibly being alive) could be only him, which creates a tons of plotholes, but at least it was possible that he survived.
So this one is not on Disney but on Rian Johnson in my opinion. That doesn't enough to defend the return of Palpatine though, because they managed to bring back Dart Maul after all! And he was cut in half AND dropped into an endless chasm! Surely they could have done something about Snoke as well!