Fantastic Beasts: The Crimes of Grindelwald blew the collective mind of the Potter faithful when it was revealed Credence Barebone is a long-lost Dumbledore. Grindelwald claims he's a brother of Albus', but this doesn't make sense to fans. Some are wondering if Grindelwald is stretching the truth. Barebone might be Aurelius Dumbledore, but perhaps he's not a brother, but a half-brother or a nephew or cousin. Fans know there's one Dumbledore family member to go to for a straight answer to this mystery. Will Aberforth be in Fantastic Beasts?
Aberforth Dumbledore is the younger brother of Albus, born only a year or two after him. Fans meet him towards the end of the series in Harry Potter and The Deathly Hallows, where he spills a whole lot of tea on the inner workings of their family, including how their mother died, what happened to Ariana, Albus' friendship with Grindelwald, the works.
Who better then, to explain how in heaven's name there came to be a Dumbledore child on a boat to America circa the year 1900? Who else could tell viewers why the baby was sent away, and who it was the family believed drowned all these years, when actually he was switched by Leta Lestrange with her own brother, Corvus.
Aberforth seems like the one to know the truth about Aurelius Dumbledore. There are two ways the next movie can go about explaining him. The more likely choice is the timeline will shift to accommodate his birth. Aberforth can then come forward and explain his older brother's wretched behavior and the death of Ariana caused their family to want to send the baby somewhere safely away.
Another choice is this is their half-brother, born of Percival while he was in Azkaban. With Aberforth being the only reliable family member left after Ariana's death, it would make sense it would fall to him, and perhaps one of Percival's relations, to get the baby somewhere he might be adopted and taken care of. This would tie into the theory the woman who drowns with the real baby Corvus was not a nanny to the switched baby, but an aunt.
Someone needs to step forward and explain the existence of this child. Under normal circumstances, Albus would explain why Grindelwald is running around with this random wizard he's claiming is a Dumbledore. But if there's one thing Rowling has established across both series, it's Dumbledore is not now, and will never be a straight shooter. Albus can't even admit to Scamander what the magizoologist needs to get from Grindelwald to free his old professor from being sidelined in this war. Newt has to intuit why Dumbledore "can't" move against Grindelwald, see the blood pact necklace, and put two and two together.
Lucky for everyone, Newt is far smarter about reading people than he'll ever give himself credit for. But audiences cannot sit around and intuit where Aurelius came from via Albus' dropped clues. Viewers need exposition, and Aberforth would be the perfect person to deliver it in Fantastic Beasts 3.