Yeah, I told you so: Kylo may not be trained enough to recognize Skywalker blood, which would answer that Vader-Leia plothole from Episode IV that was created by the reveal of their relationship in Jedi.
Or Obi-wan’s granddaughter (which is a real proposed theory) or a second coming of Anakin, or, as another version of the “born of the Force” theory has it, a completer of Anakin’s mission (or that the Jedi misinterpreted the prophecy, and there are two “chosen ones”). Doesn’t mean it’s going to happen.
This is why I disagree with @garjones that mysteries don’t work anymore. If done correctly, there can be multiple theories (ie. Luke’s daughter, Kylo’s sister, even fringe theories like Obi-Wan’s daughter or the three variations of the “born of the force” theory, or the cloned Vader theory) and at least one camp will be surprised, and the “correct camp” will have had doubts based on the strengths of the incorrect theories.
A mystery done incorrectly, is, as much as I love the books, from Harry Potter. At the end of Book 6, Harry discovers that the locket Horcrux was fake, and replaced by a “Mysterious” R.A.B. However, most Potterheads (including myself- remember I was just 11 when the 6th book came out) figured out within a few weeks of the 6th book coming out that it was Sirius’ brother Regulus Black, who must have had a middle name beginning with A (Arcturus, as it turned out) based on the evidence that Regulus was described in the fifth book as a Death Eater (follower of Lord Voldemort) who was killed because he became disenchanted with Voldemort’s extremism and got “cold feet”, which was clearly true of R.A.B. and Rowling mentioned a mysterious locket in Sirius’ family home also in Book 5.