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The Fizban Subclasses were 13 months, actually, but the Races were closer to the final release. But March is totally plausible for a September or November release, based on prior precedent, even for a Subclass. Race stuff tends to be a quicker turnaround. i think we will see Spelljammer in a few months, and Dragonlance in either September (as the yearly AL Adventure book revisiting the War of the Lance, with Setting elements) or November (if it's a more traditional Setting book).
I'm fairly confident the Spelljammer book has a strong adventure element as well. A while ago, Perkins teased the cover of an upcoming book, and it depicted Boo (the miniature space hamster) and a beholder. Although it could just be a setting book, I am leaning on it being an adventure with Minsc/Boo and something to do with the beholders in Realmspace (or maybe beyond).
Kind of like Ghosts of Saltmarsh/Wild Beyond the Witchlight/Descent into Avernus, having rules for new races and Spelljamming vehicles, in the context of an adventure.