Albus Dumbledore and Gellert Grindelwald will duel in Fantastic Beasts 3's second trailer confirms the pair will come face-to-face on at least a couple of occasions.
The Fantastic Beasts: The Secrets of Dumbledore trailer shows Dumbledore and Grindelwald having a sit down meeting, talking about the paths each is headed down and referencing their past. That could well be fascinating for Fantastic Beasts 3, given the franchise has so far largely avoided their relationship and the feelings Dumbledore had for Grindelwald. Perhaps even bigger, though, at least in of the movie delivering Wizarding World action, is the tease of a duel between the pair.
Although only a brief flash, the Fantastic Beasts 3 trailer has Grindelwald and Dumbledore casting spells at each other, which represents a noticable change from what the movies have established so far, suggesting that their blood pact has been broken. That isn't impossible, given Dumbledore himself was looking for ways to break it, but even bigger is the change it makes to Harry Potter canon. That did reveal a duel took place between Dumbledore and Grindelwald, but not until 1945 (linking Grindelwald's downfall to World War II ending). Fantastic Beasts 3 is still in the 1930s, so some way off the timeline of their known duel, hinting at another Harry Potter retcon.
It's not implausible, of course, that Dumbledore and Grindelwald had another duel prior to their 1945 battle, with enough wiggle room in the timeline and known history to allow for it to happen. However, even if the retcon does fit, it does create a risk to the story behind the duel. It's largely accepted that Dumbledore delayed facing Grindelwald until 1945, likely due to a factor of reasons including his feelings for him and, even more so, the threat of learning he was the one who killed his sister, Ariana Dumbledore (as well as the blood pact, though that may now be moot). Given how powerful and heroic Dumbledore is, then his reasons for delaying made his backstory even more tragic and ultimately help to strengthen his character arc. Having them duel before then could risk weakening that, robbing some of the meaning and impact from it.
History doesn't seem to Fantastic Beasts: The Secrets of Dumbledore's duel, so it may only be a brief encounter in the movie itself, or it may be that Dumbledore and Grindelwald's later duel was so iconic (and important), but that itself creates its own issue: if the duel isn't big enough to be memorable, then why include it when it poses a risk to what's been established? Part of the problem may lie with the Fantastic Beasts franchise's struggles - with a five-movie plan in place, that would presumably conclude with the known Dumbledore vs. Grindelwald fight, there's no real guarantee it's going to get there. Having it here, then, could be a means of ensuring a duel between them happens onscreen. Fantastic Beasts is also no stranger to Harry Potter retcons in order to serve its own story and needs, and this could be the latest example of that.
