As someone who avoided all spoilers after the teaser trailer (meaning all I knew was that Palpatine was somehow returning), I was shocked how the Palpatine reveal was so abruptly dumped into the title crawl. It felt like the trailers were being treated as part of the actual story to save time. But if you watch just the movies, it comes out of nowhere!
I love DominicCobb’s alternate title crawl. It saves the reveal for the actual film, when Kylo-Ren finds him.
Another option, as many have said, is to plant the seed at the end of TLJ. One thought I had is that so much is made of the Resistance sending out a signal for help, and no one answering. It’s reinforced again in TROS. What if they did get an answer, from Palpatine? The radio crackles, and maybe we just hear his iconic laugh. Just a tease at the end to build up anticipation for TROS.
I’m not sure how you’d do it visually. There are a few shots on Crait of the radio and Leia looking disheartened. There’s the shot just before the door is blasted where Leia is looking around the base and the radio is saying “the cannon is warming up now”. The look on her face would also work for the shock of a surprise transmission from Palpatine. You’d have to move the shot to after the climax, though, as you can’t interrupt TLJ story with Palpatine. I don’t know how well that would cut together.
Kylo-Ren is also in the radio room later where he finds the dice. Maybe that’s the time to do it. You hear some crackling and cut to closeup of the radio, where Palpatine’s laugh starts coming through. Then we cut to a closeup of Kylo-Ren with either a look of surprise, confusion, or a simple head turn, reinforced with Palpatine score. This would actually match up well with DominicCobb’s new title crawl, as it provides motivation for Kylo-Ren to search “for the source of his former master’s dark power”.
I have an even better idea: replace the rest of the end credits music with Darth Sidious’ speech from Fortnite, and before we fade into the Lucasfilm logo, we’d hear his laughter from the teaser for The Rise of Skywalker.