The Hogwarts express UCS-style was "leaked" on the 1st as an April Fools joke. The irony would be almost too much it that turned out to be true 😂. I personally think that the D2C won't be as directly connected to the anniversary sets of the rest of the year. I mean we're getting a Hogsmead, and that didn't come up until PoA, so this wave doesn't strictly need to be connected to the first two books/movies. I personally think that it'll likely be Gringotts for two reasons. 1st being that it connects to PS and is a rather iconic moment for Harry, as he realizes the full gravity (and joy!) of what being a wizard will mean to him. 2nd I believe that the designers known that Diagon Alley isn't complete without Gringotts, and they would release it soon after while the hype is still high for DA. I certainly could be wrong, it just makes sense to me!