From the looks of it, that new 8-core Coffee lake CPU, might work fine on the Z370 chipset. So Intel, while still releasing Z390, might simply be rebranding the Z370 and allow motherboard manufacturers to pair some 3rd party controllers with it for more features. 

Intel on its website recently published a page on the Z390 chipset (by mistake), including a chipset block diagram showing all connections. Z390 will support all Coffee Lake generation desktop processors but would also be compatible for the next-generation. The new rumor is that the Z390 chipset would remain the same as the Z370 chipset, manufacturers who want to offer the support that the chipset originally promised, will have to revert and do this in a more traditional way: with external controllers such as those from Asmedia, reports benchlife.

Z390 has been discussed a lot, it should be a chipset intended for 8-core Coffee Lake CPUs, but these are not expected to launch anytime soon. Z390 will get an LGA1151 socket using the traditional DMI 3.0 chipset-bus (which basically is an x4 PCIe link lane up/downlink. Similar to the Z370 chipset, it'll have 24 PCI-Express gen 3.0 lanes. Also similar is storage at six SATA 6 Gbps ports with AHCI and RAID support; and up to three 32 Gbps M.2/U.2 connectors. LAN remains the same as well. 1 GbE, Intel recommends their Wireless-AC 9560 card for the motherboard manufacturers to pair this chipset with for 802.11ac and Bluetooth 5. 

Above Intel Z390 as leaked earlier at Intel

Above (for comparison) Z370 - the differences really one are USB 3.1 gen2 and Intel Wireless-AC (CNVi)

Z390 chipset now rumored to get rebranded from Z370