We’d floated the idea of Corellia, and that’d be my pick for an extra-saga planet. But I think it’s better to keep it in-story.
Unrelated: it might do for someone to simply keep Palpatine’s face in the dark (remove flashes of lightning) until the line “as all the Sith live in me” finishes. I may be able to myself without any FX needed, so I’ll give it a try if I am able to sit down (again, that’s kneel) at the computer later.