i have to make a quote from the jedi code, "there is no dead there is the force"
based on that quote i can only say that even valkorion was defeated he has only become one with the force. and a character as powerfull as vitiate/tenebrae/valkorion could return, we've seen he's first return at the end of shadow of revan (wich is actual a good example that mere might won't fix things, it only makes them worser.) and Darth marr stated that the emperor's presence in the force has always been deceptive, so you could say that if doesn't want to be noticed, he will not be noticed, you have to note that every time he "died" was a cause of ripples in the force.
and to be honest: if he is/was 1500 years old you could think that he learned a way to mask his presence in the force, even after death (masking yourself in the force when you're merely a forceghost would be quite impressive)
and since disney is already bending the rules of the star wars galaxy it couldn't hurt anyways. and remember one of his(of valkorion) last sentences: "think of me when your alliance burns". what have we seen after kotet? a trail of events that are slowly cracking the alliance... it could end in a large event with a lot of deaths... something similar to yavin 4 or ziost or nathema... or an event that could explain why the unknown regions of the galaxy are unknown, even in the canon timelines of the movies