Given how things ended in The Last Jedi and the death of Carrie Fisher, here is my idea of how Episode IX will start. I don’t really have an idea on how it would end, but it doesn’t really matter since it’s unlikely they’ll do anything like this.
Anyway, I’d say Episode IX will be set at most two years after The Last Jedi. Rose and Finn will probably have a baby, maybe with another on the way. The Resistance just has a handful of transports and fighters and are probably hidden out in some asteroid field. (I’d love to see the remnants of Alderaan, but they probably wouldn’t do that.) Poe will be second-in-command, after Leia. Leia will be off meeting with a group somewhere trying to get them to join. There will probably be a couple staticky transmission on her progress. Possibly during one she’ll be talking and then there will be background explosions and someone shouting, “They’ve found us.” She’ll look at Poe and say, “May the Force-” and then the transmission will end. I know, I know, killing Leia off-screen in a transmission will offend people. But killing her off between the movies, saying that she’s retired and no longer part of the Resistance, saying that she’s off on some mission which we never see, hiring a new actress, hiring a new actress but CGIng Fisher’s face onto them … whatever is done will offend someone. Also, it would probably make sense that Chewie would be flying Leia around in the Millennium Falcon, maybe with C-3PO, but killing them all off-screen would be too much. Anyway, fearing they may have been discovered, the Resistance moves on.
Meanwhile in the First Order, Hux is having a ball reforming the Empire and crushing any dissent. But while Hux needs to take a cold shower whenever reading they massacred 10,000 civilians on some planet, Kylo is bored with the day to day tyranny. That’s why I said it would probably be no more than two years because that’s probably how long before Hux tries something stupid with Kylo and is killed. Kylo is focused on his White Bantha, Rey.
Rey isn’t with the rest of the Resistance. She is probably out spreading the message of the Force is for everyone. At first I thought she might be going around with Chewie in the Falcon, but she probably realizes that the Resistance needs it more than she does. This might lead to some humor over whether or not Chewie will let Poe fly the Falcon. Rey travels the galaxy by hopping freighters, maybe earning her passage as a mechanic. Never knowing where she’ll end up, she lands on a planet and goes out and talks to the people.
She ends up back on Jakku. She probably goes back to her old home, only to find someone else is living there. Maybe she goes off to some hidden cave and recovers a box of little treasures she had hidden years before. She probably has a long discussion with Force Luke. And then her old boss sees her and calls in the First Order.