This is going to sound cruder in writing than it does in my head, but I was thinking about this, and Rian Johnson killing Luke in The Last Jedi actually provides a way for J.J. Abrams to use Leia by proxy, if he feels that it is necessary to have something come to Rey by way of her. Assuming that Leia also dies, then she and Luke would, hypothetically, be reunited in death. It makes sense that Leia wouldn't have enough experience with the Force to project herself as a Force ghost (her use of it in The Last Jedi notwithstanding, which made sense to me as an instinctual emergency response, she hasn't been particularly attuned to the use of it.) So, that explains, in-story, why she wouldn't be able to make herself visible. However, I assume that Luke will be involved in Episode IX in a Force ghost capacity, probably in the same sort of cameo manner in which Obi-Wan/Ben was in The Empire Strikes Back and Return of the Jedi. If so, Abrams and Terrio have the opportunity to use Luke to speak for Leia in passing a message to either Rey or Kylo Ren or both. ("Leia wanted me to tell you....", etc.) This obviously wouldn't be their first choice if Carrie Fisher were still with us, but in terms of allowing Leia to sort of by proxy have a conversation with either of these two characters, I think it would work, if there is some piece of information that they believe is essential to come from Leia.