According to a leaked roadmap posted at the AnandTech forums, Z370 motherboards will indeed be released in Q4 2017, but that isn’t all: we’ll be treated to yet another chipset in the second half of 2018, the Z390. Not much is known about it, but expect a more optimized and performance-tuned solution for Coffee Lake.
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Source: [H]ardOCP – Intel Readies Z390 Chipset for 2018 Launch