My operating assumption for speculation at the moment is that the Gem Dragons will be in the Wildemount book, since it is 304 pages long, will apparently have a ton of monsters and Mercer is a 2E era fan (heck, his friend Coleville already pushed Gem Dragons heavily). Also, Stewart said the Gem Dragons would be coming early in 2020, which would fit an Exandria connection.
While we have a hearty portion of player material in the pipeline, it is still in line with Xanathar's Guide in terms of page count.