Although this is sort of a grain of salt thing, but it's rumored that Kylo will be visited in his throne room by Darth Vader, who will be shifting between his Anakin and Vader personas.
Of course this was also rumored for TLJ too, and seems in part based on concept art from "The Force Awakens". Concepts rejected from one film do ocassionally show up later on though (Especially with the Disney films in which many of the designs are heavily influenced by Ralph McQuarrie's old work for the OT).
https://www.inverse.com/article/3877...en-christensen