Interesting, news has hit the web that both Gigabyte and MSI will release X299 motherboards that are specifically (and only) working with Kaby-Lake-X, and not Skylake-X. The boards will get four DIMM slots opposed to eight. In the end that would drive prices down a bit.

So these boards are specifically designed to support only the quad-core Intel i7-7740X and i7-7640X processors. Gigabyte will release the X299 Aorus Gaming, which is an ATX based board with thus 4x DIMM up-to 4133 and three PCIe x16 slots  (16:8:4 configured) and the usuals as two m2 slots, 8s SATA3 and ALC 1220 for audio. MSI on their end will release the X299M-A Pro with thus 4x DIMMs two m2 slots, further detail on that one are still lacking.

Availability and pricing at this point is unknown.

Source: Videocardz, Tom's Hardware

Gigabyte and MSI To Release Kaby-Lake-X Specific Motherboards