The title "The Rise of Skywalker" is a subtle callback to Return of the Jedi - after all, in that movie, Anakin Skywalker's original personality and light-side allegiance returned from decades of being Darth Vader, so one might easily say that Skywalker rose within Vader.
Further, whichever Skywalker being referred to is "rising" rather than "returning", which suggests the growth of something new rather than the return of something old, fitting with The Last Jedi and its theme of learning from previous mistakes. The Jedi Order may have ended, but the Skywalker legacy stands to grow in a positive way.
The poster has a light blue Color Motif for its text, as opposed to the very prominent red featured in The Last Jedi and the classic yellow featured in The Force Awakens. Blue is a color that is associated with Jedi via their blue lightsabers, such as Obi-Wan's and Anakin's lightsabers. The blue is the color of the sky, a pun on the Skywalker name. The poster is a visual indication that things are going to bounce upwards after The Last Jedi brought our heroes down to their lowest point.
Just the very fact that Palpatine has returned in some form and may very well have continued to influence events from after Return of the Jedi, including the creation of the First Order and the planet-killing Starkiller Base. Anakin Skywalker's redemption and sacrifice plus all the lives lost and homes destroyed in the galaxy-wide war in the original trilogy may have been in vain.
What kind of horror has Palpatine turned into, and what extent do Rey and company have to go to get rid of him for good? Throwing any other human down the reactor shaft of a Death Star that was then blown up would leave them Deader Than Dead - but whatever form Palpatine is in, he's well enough to laugh.