Speculation…
Shmi “There was no father. I carried him, I gave birth, I raised him. I can’t explain what happened.”
Palpatine using abilities “some consider to be unnatural” conjured this child of the force within Shmi (a random slave of no importance). Palpatine is secretly Anakin Skywalker’s father (and acts like it throughout the PT).
Palpatine - having survived his fall down the Death Star chasm (much like Luke survived the Cloud City chasm - it rhymes!) used those same abilities to create Rey, again within a random junker of no importance. Palpatine is Rey father, she’s of the same line.
Seems at least possible?
Rey would have only been conceived a decade or so after the Battle of Endor. I think it’s more likely that Palpatine won’t literally appear but rather that our heroes will be dealing with some kind of contingency plan he set up before he died. After all, the books and comics have been dropping breadcrumbs to that effect since Shattered Empire and the Aftermath trilogy.
It could simply by a cameo by McDiarmid as one of the sentinel droids; https://starwars.fandom.com/wiki/Sentinel_(Contingency)
Though if they end up visiting the wreck of the DS2 I think some kind of Force Vision makes the most sense.
I agree, it might not be Palpatine in physical form… but we haven’t yet seen the Sith equivalent of a Force Ghost have we?
A Sith Wraith perhaps? (SW has never been shy pinching bits and bobs from other mythology and that would be rather Sauron like).
The only canon appearance of a Dark Side Force “Ghost”, to my knowledge, was in the Clone Wars series. And that wasn’t really a proper Force Ghost, but more of an after-image in the Force.
So far the new canon has been avoiding Dark Side Force Ghosts, but they could probably change the already pretty vague rules if Abrams wants a proper Force Ghost.