Leia's death will be announced in the opening crawl. Either the film proper starts with her funeral, or it's already been a while since her death (I'm estimating a 5-10 year time jump between VIII and IX). The only things that were really left for her to do storywise was to rebuild the Resistance, finish grooming Poe into her replacement, and have a face-to-face showdown with Kylo Ren. IX will open with the Resistance back from the grave with Poe leading it, so that much is covered.
As for Kylo... well, my take is we are going to find him seemingly triumphant - the First Order rules the galaxy, he's declared himself Emperor, he sits on Palpatine's throne or something similar - but the family he killed along the way preys heavily on his increasingly psychotic mind. (Even more so if he's the one who actually killed Leia.) If we absolutely can't use any image of Leia as a ghost, to go along with Luke's... well, he only needs to hear one echoing ghostly thing from Leia, and it's a line of dialogue we already have: "I held out hope for so long... my son is gone." Hux and Phasma (of COURSE she didn't die) will have their hands full trying to work around Kylo's madness, run the galaxy and try to counter the rebounding Resistance.