First up, the X299 successor will reportedly be named Z399. Part of the reason here is because AMD is already using X399 for the new Threadripper chipset. The Z399 will reportedly be launched alongside a Skylake-X refresh, and will not part of the Cascade Lake-X platform. The maximum number of cores for Z399 will be 22. You can expect these new processors and Z399 motherboards sometime this Fall, perhaps as early as October.
The Cascade Lake-X series, which will include a 28-core processor, is expected in 2019. TechPowerUp speculates this platform may feature the X599 chipset. So basically, the X599 will be the "true" HEDT, while Z399 looks confusing.