Harry Potter is one of the most adored stories in human history, so there has never been a shortage of fan theories about the wizarding world. J.K. Rowling's magical universe continues to expand from Newt Scamander's perspective with the Fantastic Beasts series. The trailer for the Fantastic Beasts sequel dropped in March 2018, and since then, there have been a bevy of Fantastic Beasts: The Crimes of Grindelwald fan theories apparating right onto fans' screens.
Most of these (varyingly) believable theories about Fantastic Beasts 2 connect the prequel series to the story of the Boy Who Lived proper, as there are a ton of Harry Potter references in Fantastic Beasts and Where to Find Them. However, not all of this speculation relates to Mr. Potter or his classmates.
Given the historical significance of the era in which these films take place, it's no surprise some The Crimes of Grindelwald predictions are based in real-world, muggle history. Most fans, however, desperately want to uncover the connections between these disparate wizard eras, so prepare to read a lot about Voldemort and other elements thus far exclusive to the original series.
Much has been made of the apparent apparition of wizards onto the Hogwarts grounds in the trailer for The Crimes of Grindelwald. While many believe this to be a plot hole or oversight, Redditor /u/murse_joe has an alternative theory:
"We see from Dumbledore himself that [apparating onto the grounds is] not actually impossible, he manages it. He may be just that powerful, but it’s not stated to be difficult; it’s stated to be impossible. I think Dumbledore is the one that set up, or at least strengthened, whatever prevents apparating. He may have set up up post-Grindelwald or at least post-Voldemort. Before that, people could.
Hermione says that people used to arrive different ways, one of which was apparition which often went wrong. I don’t think this was strictly the barrier, more that it was untested and untrained kids with parents, side by side is stated to be difficult too. Having students arrive by train is a side effect of the barrier, not an intrinsic property of the grounds."
There is a theory that Dumbledore's sister Ariana is an Obscurial. Redditor/u/Obversa's uses this as a jumping point for their own theory. This Redditor believes we'll learn more about the relationship between Dumbledore and Newt Scamander, specifically that the latter is working for the former in search of answers about Obscurials in order to ultimately help Ariana.
Fantastic Beasts and Where to Find Them takes place in 1926, shortly before the rise of the Nazi party in Germany. Now that Grindewald is headed across the Atlantic - given the The Crimes of Grindewald takes place in Paris - the evil wizard will be in the neighborhood as the Nazis rise to power, leading some to believe there may be a connection between Grindelwald and the Nazis, even if it's just a matter of timing.
That is essentially Redditor /u/SickBurnBro's assertion, to which he adds Grindelwald's "defeat will mark the end of the war in the muggle world." Some actually ponder the possibility of Grindelwald actually being Hitler (at least in the fictional world), but it's highly unlikely Rowling or anyone else involved would write such a monster out of real-world history. That is way too sensitive a subject. Of course, Grindelwald shares an allegorical relationship with the fascist, but that's where the comparison ends.
Redditor /u/lugua's theory actually predicts a couple things, though not with a lot of specificity. Basically, this Redditor points to the similaralities between Credence's apparent "destruction" in the first movie and the optics of horcruxes being destroyed in the Harry Potter series. The theory is that Credence will return in The Crimes of Grindelwald because he wasn't wholly destroyed in the first movie. Furthermore, we'll learn of the relationship between Obscurials and horcruxes through Credence's character in this movie.