No god is actually new. All the Seldarine deities that have been added were pulled out of some dragon mags issues, or gods that had been killed or merged in the ancient history of the Multiverse and that have now returned (like Zandilar), they're not really new.
In any case, a single book detailing all the human gods and all the non human gods, including the dozens of new gods, would probably end up with a mere SCAG-like entry for each deity, with little to no room to dedicate to details. It would make it worth to newbies, that's for sure (which is why I'm hoping that this book will be published), but not to people who have access to (or are willing to grab) the super in-depth lore of the old books (not to mention that you can get all three on the DMGuild for far less than $50, and the lore in all 3 is mostly up to date, because the Sundering did nothing but restore all the gods that had been lost along the way, including the merged ones).
Honestly, thinking about that, the sheer amount of deities that need to be included, and the knowledge that WotC will never dedicate more than 1 book to this topic, kinda defeats the original point of having this book, to begin with. It would end up being a repetition of the SCAG and MToF sections about gods, with a few other gods getting a small writeup in addition to the ones that have already received one. It could never include the kind of info that you suggest in the op, or a detailed insight into how the clergies work (such things would be quite amazing to read in a book, but WotC isn't going to include them, realistically speaking).
Perhaps the word obscure would have been better then new, although some are new to the Forgotten Realms.