If you still haven't watched STAR WARS EPISODE VIII: THE LAST JEDI and don't want to be spoiled, it's best to click away now, because discussion for the next movie will refer to key lines and plot points from VIII.
In THE LAST JEDI, Kylo Ren exhorts Rey to let the past die...to kill it, if necessary.
What if Rey takes his advice to heart, both figuratively and literally?
The past of STAR WARS and the Force, the Jedi, lies with the Skywalker legacy. Anakin Skywalker is dead, and so is Luke. Leia lives, but for how much longer (and she's unlikely to procreate more Skywalker progeny before her time on the mortal plane ends)?
If Rey kills Kylo/Ben Solo in EPISODE IX, the Skywalker bloodline...the past...ends, and she will have killed it. Rey, whom Kylo Ren (if he is telling the truth) has plainly stated is not of the Skywalker bloodline.
Symbolically, the Skywalker lightsaber built by Anakin and wielded by himself during his time as a Jedi and later by Luke, was finally destroyed in THE LAST JEDI.
Rian Johnson has said that his eventual new trilogy, which will take place after EPISODE IX, will move STAR WARS beyond the mythos of the Skywalkers.
Based on this information, I believe Rey will kill Kylo Ren in EPISODE IX and end the legacy of the Skywalkers, starting a new chapter in the history of the Force and the Jedi.
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