I've described their method of teasing products as a "shotgun" method. There's all kinds of hints and clues just scattered about. There are the real clues that lead to the planned adventures, the clues they threw out because they might do a related adventure in a few years, the clues that are really just referential Easter eggs to past products, and the clues that are really just nothing but catch the eye.
You often can't tell except in retrospect. But there's often quite a few lucky guesses. Law of averages really...
They obviously plan a year and change in advance. Because they have to. They need that much lead time for partners like WizKids and Cryptic. But it's not like they have a rock solid plan.
They brought Matt Mercer in to plan Dragon Heist when he was doing Force Grey in 2017, and that led to some of the plot threads like the multiple villains (before that it was just Manshoon). So they had the idea of the city adventure and heist planned that far back, but it was in a much looser state. So while they may have planned the 2019 products in late 2017 and early 2018, the final product might change dramatically.
I didn't expect them to do two back-to-back adventures in 2018. Because that's a lot of work for poor Chris Perkins. But, of course, he went and relied on freelancers for a lot of the heavy lifting, and had a shorter L1-5 adventure to start). Still... three adventures from the D&D team in a row still seems like a stretch when they could instead do two sourcebooks separated by a summer adventure. (Possibly getting back to the two-adventure rotation of 2020.) But, again, a reprint collection could work, being easily done by Mearls and Crawford.
Since we haven't seen much in the way of playtest content, and the shortest turn around they've done is three months between the August Ravnica races and Guildmaster's Guide to Ravnica, I don't think the spring release will have much in the way of PC content.
Or it could be a campaign setting.
Or it could be another monster book.
Or something else altogether...