While we knew before the first movie came out that it would be the first of five films, it wasn’t exactly clear at the end of Fantastic Beasts and Where to Find Them what the next step in the story would actually be.
Based on the comments of the tweet that J.K. Rowling is responding to, it seems that the spoiler info may simply relate to where things are left at the end of Fantastic Beasts: The Crimes of Grindelwald.
All we really know now is that Grindelwald will continue to be the antagonist, but it sounds like we may have some more specific direction at the end of the new film, a direction that may be entirely unexpected.