In an unnoticed September 2021 interview with Metro (dug up today by r/starwarsleaks), Ian McDiarmid was leaving the door open to his return as Palpatine/Darth Sidious in future Star Wars projects:
“I think I have to accept that, thanks to my ungrateful granddaughter, my annihilation was finally final. [But] of course [Darth] Vader is back in the new [Obi-Wan Kenobi] television series so I suppose it’s not impossible that one day his mentor might be discovered once again skulking in the shadows.”
I really hope that he secretly filmed some scenes for Obi-Wan Kenobi with Hayden Christensen as Darth Vader. McDiarmid must be part of this Prequel Trilogy reunion!