Force Ghosts appear and help in final fight
The Rise of Skywalker will definitely have nods to every film that has come before it. A logical way to include past Jedi would be to have them appear as Force Ghosts to help out Rey and perhaps Ben in the final fight against Darth Sidious.
Prior to the final fight, it has to be assumed Force ghost Luke will make an appearance or two to help instruct Rey. So we’ll get a dose of Luke before the end. In the final battle perhaps Sidious can only be destroyed by using the Force and our new heroes will need some help from some old heroes. It wouldn’t be shocking to see Anakin, Yoda, Obi-Wan, Luke, and maybe even Qui-Gon Jinn show up and use the Force to put an end to the evilest and most powerful Sith who ever lived.
It will be revealed Sidious created the Skywalkers
This has been hinted in Revenge of the Sith but in Episode IX we will get a definitive answer. It will be revealed it was Darth Sidious who controlled the midichlorians and created life in Shmi Skywalker. He will reveal this information to help keep Kylo Ren on the Dark Side of the Force.
In a recent TV spot, the voice of the Emperor can be heard as he calls Kylo Ren “My boy”. Yes, that can be just a general term, but what if he means it literally.
If he did in fact, create Anakin Skywalker then he’d be his father, and Ben’s great-grandfather in return, and he would be “his boy” to an extent. Knowing he was created from the Dark Side could be what it takes to kill off Ben Solo for good.