My thoughts on Leia--
Movie begins, crawl explains the new Rebellion continues to fight. Her message that went unanswered in TLJ has taken flight and more people are pledging alliance. An old Alliance general named Lando meets up with Poe to join the cause and he wants to see Leia. Poe takes him to their secret baee and leads Lando to a grave.
Leia died between films, but she's been kept alive in voice/holograms and become an icon and symbol for the movement, though her death has been kept secret.
I think something like this would be cool-- it honors the character, doesn't require recasting, dismissing here as "not there," or cg trickery, and still advances the story without getting clunky. FX wise, it would be pretty easy to recycle existing TFA/TLJ footage to create a hologram Leia-- in fact, Poe could even be literally doing that-- editing old recordings of her to keep her voice and message alive.