I liked the concept they almost went with in TFA, having Anakin shift between the light and the dark in his appearance. He might also be the only one Ben might listen to, given his Vader worship.
It would be really disturbing if a cast off dark side persona took on a life of it’s own. A Vader apparition without any good left in it.
It’s an interesting idea to have a Anakin/Vader hybrid ghost, but it’s a little too far fetched for me. I think you could introduce Dark Side Force sprits, but I believe it needs to be done right, rather than doing it just because it seems cool. Very much like why Lucas gave Yoda a lightsaber in the prequels. Plus, Anakin’s already been redeemed. Having a hybrid Light and Dark side ghost would feel like backtracking for Anakin as a character, especially since he’s now dead.