The teaser trailer dropped two major revelations amongst its footage: the title, which is The Rise of Skywalker, and the return of Emperor Palpatine, with a laugh that sent shockwaves around the internet.
The return of Palpatine has, quite understandably, led to the rise of fan theories as to exactly how he'll come back, but an equally pertinent question is why. In-universe, Palpatine has been dead for over 30 years. The remnants of the Empire were either wiped out, or fled to form what would become the First Order. We've already seen one Emperor-esque figure killed off in the way of Supreme Leader Snoke, with Kylo Ren stepping up to ostensibly become the new Big Bad, so why is Big Sheev back?
Palpatine's return allows the saga, at its end, to connect all nine Skywalker movies, not just Episodes IV-IX. That could allow it to revisit Anakin Skywalker's story, because the saga belongs to him as much as it does Luke. And with J.J. Abrams at the helm, it could also revisit Rey's parentage. Biggest of all, the two things might be related, with Abrams to tie it all together in rather shocking fashion, by having Rey as the creation of Palpatine, and his attempt at making another Anakin.