Yes, it’s almost entirely dependent on finding the right actor. I’m debating about making them a known but previously unseen character from Star Wars lore, like Plagueis. This would give him an immediate alien appearance which would visually confirm the lie of Rey being his offspring as well as continuing the ST trend of including some aliens in with the otherwise human villains.
JEDIT: Since Palpatine in this film is hanging from a gantry and has withered useless hands, it would make sense for the alien mastermind to be the reanimated corpse of Snoke. He’s a Force-sensitive alien, after all, so who knows if he would have actually died to bisection anyway. Maul apparently didn’t. Since he also lost his hands in the bisection, his gaining his hands back would be a real plot point, as well as his hatred for Rey and Ben. If his face is in shadow for most of the scene, emerging only at the end to morph into that of Palpatine, it would be fairly simple to use doctored footage of Snoke from TLJ and merely have an actor who could do a passable Snoke impression.
On to the scripting. We begin with Han and Ben’s reunion:
Ben: “I know what I have to do, but I don’t know if I have the strength to do it.”
Han: “You do.”
Ben: “Dad.”
Han: “I know.”
Establishing shot of Sith Temple on Exegol, cutting to a conference table within the Evil Lab. A hooded figure with a shadowy, alien visage sits at the table, surrounded by the holographic images of the Knights of Ren.
Knight: “We’ve lost contact with Leader Ren. We think he’s gone rogue.”
Figure: (With a voice which is labored, twisted, alien) “This is the princess’s doing. But it is to our advantage; the First Order is mine once more. And when the girl arrives, we shall use her power to destroy the Resistance.”
Knight: “But if they take out the holographic emitter, most of our fleet will vanish into thin air.”
(The figure generates a schematic on the conference table indicating the emitter which is generating the images of thousands of Star Destroyers among a scattered few real ones)
Figure: “General Pryde’s destroyer is capable of sustaining the illusion. Rest assured, the Resistance will believe that it faces ten thousand devastating warships. Without hope, and with the return of their ‘Emperor’, the galaxy will fall to the Final Order.”
Knights: “Long live the Final Order and the Sith Eternal.”
Knight: “And what of the girl? If she doesn’t take the bait…”
Figure: “She will. I am her grandfather, after all.” (He emerges into the light. An inhuman, grinning face, the face of Snoke reanimated.)
Knights: (sinister amusement)
Figure: “Return home, my brethren. All will go as planned. And if the last of the hated Skywalkers dares to interfere with that plan…(his voice changes)…do what must be done. Do not hesitate, show no mercy.”
The face of Snoke morphs into that of Palpatine beneath the hood.
Esablishing shot of a First Order destroyer.
Palpatine: “The Princess of Alderaan has disrupted my plan. But her foolish act will be in vain. Come to me on Exegol, General Pryde.”
Pryde: “As I served you in the old wars, I serve you now.”
Palpatine: “Send a ship to a world they know. Let it burn. The Final Order begins. She will come. Her friends will follow.”
Pryde: “Yes my lord.”
A Sith Eternal destroyer fires weapons reminiscent of those used on D’qar at Kijimi. The city centers are ravaged, but the planet is not destroyed.
“Kijimi? How?”
“A blast from a Star Destroyer. A ship from the new Sith fleet. Out of the unknowns.”
“The Emperor sent a ship from Exegol.”
“This is how he finishes it.”
“Listen, it’s on every frequency.”
“The Resistance is dead. The Sith flame will burn. All worlds, surrender or die. The Final Order begins.”