One of the truly great villains in Star Trek history is the nefarious genetically engineered superman from the late 20th Century, Khan Noonien Singh. Portrayed in the original series in 1967 and the follow up film in 1982 by Ricardo Montalbán, he was one of the few villains who could out-ham William Shatner.
His dramatic demise in The Wrath of Khan managed to take out Spock in the process, and set in motion what would be the plot for the next two classic Trek films.
Word leaked early that Khan might be the villain in Into Darkness, and early rumors had Benicio Del Toro (later of course “DJ” in TLJ) trying out for the role. But when Benedict Cumberbatch was cast, it seemed to reduce the likelihood of Khan’s return.
This year at Star Wars Celebration in Chicago when The Rise of Skywalker trailer debuted, actor Ian McDiarmid was there after the voice of Emperor Palpatine-Sith Lord Darth Sidious-closed out the footage.
Abrams is keeping his cards close to his chest still regarding how Palpatine will return (and if Matt Smith is involved), but teasing this up front seems the director is trying to avoid some of the backlash he got regarding Khan. Abrams is known for what he calls his “mystery box,” building many of previous projects almost as puzzles to be solved; Lost, Alias, and to some extent The Force Awakens seem to play on this.
This time Abrams does not seem to be deliberately misleading us with his choice of villain, but making the ‘how’ the mystery instead of the ‘who.’
That being said, Khan as portrayed in STID did some pretty deep dives into the existing character story, right down to matching the number of survivors from the 20th Century Eugenics War.
Expect any appearance of Palpatine under Abrams to be explained with some deep cuts into the existing lore of the Force, the Sith, and certainly some of the new canon stories of what happened in the days during which the Empire was falling.