It's no secret that George Lucas took a page from Joseph Campbell's
The Hero with a Thousand Faces when crafting the basic plot line for the original trilogy. Campbell's thesis was that the Hero's Journey (diagram above) is the plot for practically every compelling myth we know of, whether or not we're aware of it. Applying this to the Disney trilogy, I have a theory about how both Kylo and Rey are both the heroes... but that Kylo begins his journey a further along than Rey. This has clear implications for what Episode IX should be like. So, I marked the diagram with clock numbers to make this easier to explain. Here's the basic gist --
In TFA, Kylo starts at 4, he's a character who is already beyond the initial threshold. By the end of the film, he reaches 6. He kills Han Solo, which is the deepest/darkest moment in his life. In TLJ, he's the new and improved Kylo, he starts at stage 7... and his killing of Snoke is his act of redemption and atonement. What does this mean for Episode IX? He's going to come back home, obviously, but it might not be exactly what we have in mind. He'll start at 10, and move to 12 by the end of the film. As the Supreme Leader of the First Order, it's not like he can walk away from it all without paying some sort of price. Nor do I think the Resistance (or the rest of the galaxy) will welcome him back with open arms if he returns to the Light Side. He will have to either (a) die, (b) willingly destroy the First Order from the inside out, or (c) go into exile and rebuild the Jedi Order. The question is, which of these sounds like a Call to Adventure? My guess is that some combination of (b) and (c) will be the case. I imagine the final scene being something like
Kylo Ren Ben Solo onboard the
Millennium Falcon, with Rey perhaps, embarking on some new adventure.
Now onto Rey. She more appropriately fits the mold of the Journey by starting out at 12, but there are some twists. By the end of TFA she passes the Threshold (defeating Kylo Ren) and connects with Luke (the Helper/Mentor), stage 4. In TLJ, she passes her temptation (Dark Side cave) and reaches the Abyss (learning the truth from Kylo about her parents). In Snoke's throne room, we see a transformation, but I believe it is a fake one: she is pretending to be a Jedi because she's stuck on this good vs evil and Jedi vs Sith trope, which was what Luke was trying to steer her away from. So, I think the movement of Rey (as a character) in IX is going to be somewhat blunted or completely stall. We're going to be left with a transformed "Jedi Knight" Rey, who will never truly make it to 9, leaving 10-12 out of the question entirely. She'll stop at 8 by the end of the film, and that'll be that.. because there is nothing to atone for. This could have several implications for Rey in IX. One possible plot is that because Rey's transformation was false and/or her character has peaked early, she will have to go backwards:
(6) she will experience another abyss - probably the death of Leia
(5) temptation - probably giving into the Dark Side because she doesn't see the dichotomy between Light/Dark and is more so a Gray Jedi
(4) mentor - probably Leia
(3) threshold - battling Kylo one last time
(2) guardian - Luke's force ghost will show up
(1) supernatural aid - probably all the force ghosts will show up
(12) adventure time - riding in the Falcon
Anyway, that's my theory and I'm sticking to it!