Everyone's least favourite addition to the canon might be making a comeback. The concept of midichlorians was introduced in The Phantom Menace as more concrete, scientific explanation for the Force (subatomic particles that acted as the liaison between the Force and matter, not so different from our real-world Higgs-Boson particle). It was met with pretty negative backlash as many felt it was an unnecessary explanation for the spiritual elements of the series. But midichlorians were further explored in Revenge of the Sith when Palpatine subtly hinted that he and his master harnessed the power of midichlorians to create human life (specifically Anakin). If Rey is not a Skywalker, the theory that she is some sort of clone or artificial creation of Palpatine (who we know from the Aftermath book series had a secret base on Rey's home planet of Jakku, aimed at clandestine scientific experiments) is starting to gain a lot of traction. If Palpatine is talking to her in the most recent trailer, it certainly lends itself to the theory.