The casting of Fiona Glascott as the young Professor McGonagall in Fantastic Beasts: The Crimes of Grindelwald has broken Harry Potter canon. According to Harry Potter lore, McGonagall shouldn't have even been born in 1927, when Fantastic Beasts 2 is set.
The Fantastic Beasts films are set decades before Harry Potter was even a twinkle in his mother's eye, let alone before he walked through the doors of Hogwarts for the first time. At the same time, though, they're increasingly seeming to be focused on the backstories of major characters in the Harry Potter books and films. Fantastic Beasts 2 features Jude Law as Dumbledore, a number of characters who are typically associated with the Death Eaters, the alchemist Nicolas Flamel, and even Nagini, the cursed woman destined to become Voldemort's snake. Unfortunately, the simple truth is that the more J.K. Rowling attempts to connect the Fantastic Beasts story to Harry's, the more risk there is of damaging - or, even worse, breaking - continuity.
That moment seems to have finally arrived. The latest reports suggest that Fiona Glascott will play Professor McGonagall in Fantastic Beasts: The Crimes of Grindelwald. Given the actress is in her '30s, it's reasonable to assume McGonagall will be working at Hogwarts alongside Albus Dumbledore. This causes a massive continuity problem for the franchise; Fantastic Beasts 2 is set in 1927 - eight years before McGonagall is even supposed to be born.
The Harry Potter books themselves only offered hints as to Minerva McGonagall's past. The most interesting was in Harry Potter and the Order of the Phoenix; in one scene, McGonagall was asked how long she'd taught at Hogwarts. "Thirty-nine years," McGonagall replied. Given the Harry Potter stories are set in the '90s, that means she began teaching at Hogwarts in 1956.
But J.K. Rowling has spent the last few years fleshing out the Harry Potter universe on her Pottermore website. There's a sizable entry for McGonagall, revealing her Scottish "half-blood" heritage, an early heartbreak when she fell in love with a Muggle, and even how she began her teaching career. According to Pottermore, McGonagall graduated from Hogwarts with top marks and briefly worked for the Ministry of Magic. Disgusted by the anti-Muggle prejudice she encountered in the Ministry, she quickly decided she'd had enough after 2 years and applied for a teaching job at Hogwarts. Between Harry Potter and the Order of the Phoenix and the Pottermore website, then, it's possible to work out that McGonagall was born in 1935. She shouldn't even be alive in 1927, let alone already in her '30s.
McGonagall's backstory on the Pottermore site is remarkably detailed, which suggests Rowling never really expected to revisit the character. And that, fundamentally, is the problem with the entire Fantastic Beasts film series; these are movies Rowling never expected to script. Had she been dealing with entirely original characters, she'd would have avoided these kinds of contradictions, but every time she decides it's time to use a familiar name, she's forced to rework a character into the story of Newt Scamander, Gellert Grindelwald, and Albus Dumbledore. Sometimes that will inevitably mean that their history changes. In the case of Professor McGonagall, it appears to mean she's significantly older than Rowling originally intended.
Nobody yet knows what McGonagall's role will be in Fantastic Beasts: The Crimes of Grindelwald. All this is written assuming that she's a teacher at Hogwarts; it's a reasonable assumption, but there is just one other alternative. It's just about within the bounds of possibility that, at some point in the film, Albus Dumbledore will experience of his personal future - and perhaps glimpse Minerva McGonagall, a woman who's yet to even be born, at his side. That could conceivably happen, especially given dialogue in the Fantastic Beasts 2 trailers has already suggested Dumbledore's old flame Grindelwald has been having visions of the future; but it does seem rather unlikely. There's no evidence whatsoever that Dumbledore has ever been a Seer, a subject that would surely have come up at some time in the Harry Potter series. So, sadly, it's far more likely that J.K. Rowling has just made a significant continuity error.
- Fantastic Beasts: The Crimes of Grindelwald/Fantastic Beasts 2 (2018) release date: Nov 16, 2018