Looking back over The Last Jedi, I just realized something. What if Palpatine being the big bad to come back in Episode 9...it was Vader?
The movie still opens the same, Kylo Ren (with his mask already rebuilt, I do like how he only takes it off when he's in front of Rey) finds the Sith Wayfinder, makes his way to the Sith Homeworld. He finds the vats of Snoke clones, but also finds a vat of...something else. Around the midpoint (or possibly earlier), instead of Kylo Ren firing at Rey on the Burning Man Planet, it's Vader, in a perfect replica of his suit.
So, why Vader instead of Palpatine? For starters, as much of a Palpatine mark I am, it reduces the impossible, credibility stretching "I am the author of your pain" narrative of Palpatine, giving him too much power and presence in the franchise, which kinda makes the OT all for nothing.
Secondly, it fits Kylo Ren's character. For all his talk of "let the past die," he very much lets his past define him. He hesitated to kill his dad, he refused to kill his mom, and chose to fight Luke personally to get some form of closure. Plus, who's the person that Kylo has been hero worshiping/modeling himself after the whole Sequel Trilogy? Vader. Vader deserves to come back for Episode IX more than Palpatine. You can make Rise of Skywalker a dark inverse of The Last Jedi, where our impressionable youth is able to get their hero of old to fight on their behalf from the beginning.
However, since Kylo is too impatient to do the process properly, he opts to make a Vader clone that fits his vision of what he thinks Vader was like: cold, ruthless, powerful, but obedient, the real edgelord, badass Vader that people loved from the last five minutes of Rogue One. Maybe even add a third act twist where Clone Vader doesn't need the armor, and he's been using the cloning tanks behind Kylo's back to get back his human parts and become even stronger than The Emperor ever was.
However, as a result, this clone has no redeeming qualities, namely, his (no matter how twisted) desire for a family. Clone Vader doesn't give a damn about Kylo's blood tie to him, no does he care about Kylo's loyalty to the Knights of Ren. You know, actually CHARACTERIZE the Knights and create more opportunities to humanize Kylo. Clone Vader is all of Vader's worst personality trait cranked up to 11 because Kylo didn't bother to know EVERYTHING about his grandfather.
I'd even go so far as for as for Kylo to figure out the truth about Vader/the Skywalker lineage, being that they were a Sith creation by Palpatine, because I actually do like the idea of Palpatine playing SOME part in Episode IX, as well as the Skywalker bloodline going extinct, truly put an end to the Skywalker story.
As for needing a MacGuffin to chase, hey, maybe the Resistance intercepts an Imperial transmission about continuing the project of capturing Force sensitive children and conscripting them as Stormtroopers/ Knights of Ren (hey look at that, another intersting subplot/expansion on Force Awakens that went nowhere) and Rey, Finn, Poe, and Rose have to find them before Kylo and Clone Vader do. The film's external conflict isn't about saving the galaxy, but protecting the futures of children/young adults that could be/have been destroyed by the First Order.