WARNING: The following is just rumor and speculation, but it could be true from a certain point of view, so if you want to avoid possible SPOILERS for Star Wars: Episode IX, you should jump to hyperspace out of here!
Heading in to Star Wars: The Last Jedi, there were countless theories out there to answer the most compelling questions raised by Star Wars: The Force Awakens. Since Rian Johnson's film answered those questions (unsatisfactorily for some), there doesn't seem to be quite the same level of theorizing for J.J. Abrams' Star Wars: Episode IX.
Nevertheless, as is the case with any property this popular and secretive, there are still plenty of theories and rumors out there about what we could see in this all-important final chapter in the Skywalker Saga. So, grab those saltshakers as we dive into the Bacta tank of rumor and speculation about Star Wars: Episode IX.
Last time, we contemplated just how powerful Luke Skywalker will be in this next film. Like his father before him, Luke is a prodigy, but he didn't get to where he was all by himself. He had help, and the person who started him on his journey to becoming a Jedi Master was an old hermit by the name of Obi-Wan Kenobi. He just so happens to be the subject of a particularly interesting rumor that came out a while back.
This rumor, which surfaced back in June, was that Ewan McGregor would secretly film scenes for the next Star Wars film, i.e. Episode IX. Six months later, we don't know much more about J.J. Abrams' film, so an Ewan McGregor Obi-Wan appearance would seemingly still be possible. So let's speculate how likely this is to happen and what form such an appearance could take.
First of all, it should probably be said that of all the reasons that Ewan McGregor's Obi-Wan might not show up in Episode IX, a lack of interest from the actor is probably not one of them. Ewan McGregor, who was, according to many, the best part of George Lucas' prequels, has shown continual enthusiasm for the character and the property over the years, so you have to think that if he's asked to be in Episode IX, he'll be on board faster than you can say "Hello there."
Now it is prudent to acknowledge that Ewan McGregor has already had a part in the sequel trilogy, in Star Wars: The Force Awakens. In Maz Kanata's castle on Takodana, Rey discovered Anakin Skywalker's lightsaber and upon touching it, had a vision. At the end of the vision, Rey heard Obi-Wan Kenobi say "Rey, these are your first steps."
We don't know exactly what J.J. Abrams' initial vision for this trilogy was and if that line from Obi-Wan was meant to just be a bit of a nod to the past in the nostalgia-driven The Force Awakens. But it certainly seems within the realm of possibility that it was a tease of things to come for the Jedi Master, at least in J.J.'s mind. By returning to the franchise to close it out, J.J. Abrams might want to pick that thread back up and run with it.
Again, we don't know what that might entail, but being that Obi-Wan is dead, the most obvious way he could show up in Episode IX would be as a Force Ghost. We've already seen Yoda's Force Ghost in this trilogy,- (and likely will again), and Luke Skywalker will presumably be a Force Ghost in the next film, so why not round out the trio with Obi-Wan, who can help Rey with her next steps? If Rey is to rebuild the Jedi or restore balance to the Force, she could seek guidance from multiple generations of Masters, like Aang in Avatar: The Last Airbender.
On the other hand, including Obi-Wan as a Force Ghost, alongside Luke and possibly Yoda could make Star Wars: Episode IX feel a bit crowded and like a movie full of dead characters. It could also be considered a bit too fan service-y, and J.J. Abrams might want to avoid the same critiques that were levied against The Force Awakens.
One option that might satisfy the desire to see Ewan McGregor's Obi-Wan and provide a bit of fan service without feeling shoehorned in or being detrimental to the story might just be a simple cameo. If this film gives us a happy ending, we could get a scene reminiscent of the end of Return of the Jedi, where we see the Force Ghosts of all the departed Jedi celebrating peace finally being brought to the galaxy and balance being restored to the Force.
Seeing Ewan McGregor's Obi-Wan sharing the screen with Mark Hamill's Luke, as well as Yoda, would be a cool way to close out this Saga, bringing things full circle with all three trilogies. This seems to be the easiest way Ewan McGregor could appear in the film and perhaps the most likely. Heck, maybe we could get Anakin and Qui-Gon Jinn's Force Ghosts to show up as well.
All that said, another aspect to the lightsaber vision in The Force Awakens is that it acted as kindling to the 'Rey is Obi-Wan Kenobi's granddaughter' theory, one of the more prominent theories about Rey's parentage before Episode VIII. Now, we won't go too far down the rabbit hole of Rey's parents and whether or not J.J. Abrams will retcon what Rian Johnson did, but it does seem like J.J. had someone else in mind than junk traders. If his initial plan was some sort of Kenobi lineage, he could return to that by just making Kylo Ren's revelation a lie.
If that were to occur, Ewan McGregor could tell Rey this as a Force Ghost, or we could find out via a flashback or another vision. It is hard to say what impact this would have without knowing how it would be done, but it is feasible. If he is in Episode IX, I don't expect Obi-Wan to have a large role, yet he could still be important in this story, and being related to Rey would certainly fit the bill.
On the other hand, redoing the Rey parentage reveal is a lot of extra weight to lift and time to devote for a film that also has to see the battle between the Resistance and the First Order come to a satisfying conclusion.
The last aspect to this rumor is the specter of the Obi-Wan Kenobi spinoff movie. In the initial rumor, which offered little in the way of clarity, it was said that Ewan McGregor's appearance as Obi-Wan in Episode IX essentially meant that we won't be getting the long-rumored, but never fully confirmed Obi-Wan spinoff film.
After the financial disappointment of Solo: A Star Wars Story, we don't really know what the future holds for the spinoff films, but it doesn't look good with the cancellation of the Boba Fett movie. That doesn't necessarily mean we won't get an Obi-Wan movie at some point (maybe sooner rather than later), but Bob Iger has intimated that a Star Wars slowdown is on the way, indicating that Disney is going to be putting the bulk of its focus on the main series of films for now.
I don't think Lucasfilm would shoehorn Ewan McGregor's Obi-Wan into Episode IX to appease fans in lieu of a spinoff film. Such a move would be putting fan service ahead of story. But if J.J. Abrams was planning for Obi-Wan to appear in his film regardless, maybe that would make the parties involved feel better about the Obi-Wan movie being put on the back burner for now. If Ewan McGregor appeared in Episode IX, his appearance could also be used to gauge fan enthusiasm for a Kenobi spinoff film (hint: it's high).
Ultimately, this is all rumor and speculation, but if I had to put down my galactic credits on whether or not Ewan McGregor's Obi-Wan Kenobi will be in Star Wars: Episode IX, I would guess yes, but only in a small, cameo capacity.