Anakin’s force ghost
At the end of Return of the Jedi Anakin Skywalker comes back to the Light Side of the Force by saving his son Luke, and tossing the Emperor down the reactor core of the Second Death Star.
In that exchange, his suit is destroyed and he can no longer live without his breathing apparatus. Luke hauls his body off of the Second Death Star just before it blows up, and then burns his corpse in a pyre on the Forest Moon of Endor.
At the end of the film during the heroes’ celebration with the Ewoks, Luke steps away and gets a vision of Obi-Wan, Yoda, and Anakin as Force ghosts. Some of us still remember when Anakin’s ghost was his older version, but that has been replaced by Hayden Christensen’s Anakin due to George Lucas’ constant tweaking. That move may have opened the door for Anakin’s on-screen return.
It only makes sense for Anakin’s Force ghost to return in The Rise of Skywalker. He very well could be the Skywalker in the title and being the chosen one, it could be him who rises in the end to save the day.
Even if his role isn’t that vital, one has to think he’ll make an appearance. This movie is going to have callbacks to earlier films and characters from those films. Star Wars is basically Anakin’s story and he needs to have some role in the final chapter. As maligned as his original performance was, fans have recently softened on the Prequels and would go nuts if Hayden Christensen reprised his role as the Skywalker patriarch.