Chris Coleman, on precedent for Darth Sidious making an appearance in Episode IX:
That doesn’t mean that Dark Side users are completely out of luck. The Dark Side is a pathway to many abilities some consider to be unnatural. There are at least two recent examples where followers of the Dark Side were able to have some limited form of life after death.
A good read and breakdown. Glad Chris's blog is back for this last-go-round in the Skywalker saga.