I noticed this previously, and, though not Spelljammer related, stated that there are 17 Outer Planes for a Planescape book to cover...A question for those who know more about Spelljammer than I do: "Eberron: Rising from the Last War" details 17 locations and then has a large block of the book describing Sharn. "Van Richten's Guide to Ravenloft" details 17 domains, and then has a block of minor domains. I would add that the outer planes of the great wheel plus the outlands also adds up to 17. Does Spelljammer have 17 of anything?
Also, the Great Modron March goes through the cosmos every 289 years, which is 17 squared (which has been lore since 2e at least). Sadly, there are only 15 types of modron - making that 17 as well would have really brought things full circle!
There aren't 17 "major" crystal spheres, but I'm sure if they want to keep the pattern up, it wouldn't be hard to write up the better-known spheres and then add more on (probably classic, but lesser known, ones, but they might very well add on one or more entirely new ones) to reach that number. I would assume that the individual focus would be on the Rock of Bral, and Realmspace as a whole (although less focused on), since it's canonically located there...