I think that there are a couple of ways that this could go...
The first, and one that has been abundantly established in the canon novels and comic books, is that Palpatine was being drawn to a very powerful presence of the Dark Side of the Force that was located "out there." There has been much conjecture that Snoke was this presence, but... What if he isn't? Or... What if he's only a piece of that distant Dark Side power?
It could be that General Leia becomes tasked with finding out exactly what this new threat is, and the film ends with her leading an expedition to acquire critical information. In Episode IX nothing more is heard from her, and we're left to presume the worst. Although it's hard for me to imagine that no matter the distance, that if Leia died that Luke wouldn't feel her physical death, and we'd have to get some kind of recognition of it in one of the movies.
Another possibility, and, I'll admit, this is a STRETCH, is what if Leia does get killed in this film, perhaps even slain by her son? Now, this opens up a can of worms in this sense - if this happens, then we well know that she was supposed to play an enormous role in Ep. IX; how could this be if she's slain at the end of Ep. VIII...?
They intended that she'd come back as a Force ghost, for she had been trained to retain her consciousness by Luke, as he knew well of it from Obi-Wan, Yoda, and then even his father. In fact, from Luke's point in history, the fact that Jedi could do this would be natural, as every Force adept that he had known was capable of doing this. Leia's death at the end of VIII would've set her up to be the Gandalf The White of Ep. IX.
With Fisher's death, this Force ghost plot point would become simply impossible, and for a whole host of reasons. It becomes downright distasteful to try and CGI a Force ghost effect "actress" when the real deal is actually dead. Taboo? Completely.
Still, I think that this approach is highly unlikely.