It would make total sense for Darth Vader or Anakin or an amalgamation of the two to appear to Kylo Ren.
If Kylo Ren is to be redeemed, having Anakin/Vader show up in Kylo’s story might be one way to force him to see his grandfather’s mistakes and that he changed sides in the end.
Although he seemed to turn the corner fully to the Dark Side in the last film, Ben Solo’s struggle with the light has been central to his character throughout this trilogy, much as Anakin struggled with the dark in the prequels.