Short version of what I said above: all of the X-Series processors are exclusively packaged for what Intel calls "High End Desktop (HEDT)" platforms, and none of these ever has on-die iGPU.
Z370 (along with other 300 Series chipsets) is expected to use Socket LGA1151 (compatible with existing Sky Lake and Kaby Lake CPUs), although it may use a newer socket instead. It will run Coffee Lake CPUs, and if they are LGA1151 they should be compatible with existing 100 Series and 200 Series motherboards. After firmware/BIOS compatibility updates, anyhow, lol. Another version of Coffee Lake is Cannon Lake, basically a 10nm shrink on Kaby Lake (hybridized with some new Coffee Lake tech refinements), and it will in turn be upgraded to Ice Lake in 2019 - these are intended only for mobile (laptop) devices but desktop versions may exist. All this stuff is what Intel calls "Mainstream Desktop" or "Performance Desktop" platforms, and they (almost) always have on-die iGPU.
Googling "Intel roadmap" or reading reviews and wikis explains this in greater detail. Confirmed facts aren't yet complete - there is a great deal of speculation, exaggeration, elaboration, and fabrication to sort through - but Intel constantly "leaks" more and more tantalizing details into public domain, it's free advertising!
Any CPU compatible with Z370 (whatever socket it uses) will have iGPU. I don't really understand the hype about iGPU anyhow, it's basically just wasted silicon on the CPU die which would be better allocated towards real processing power, cores, cache, Uncore controllers, whatever. Any serious gamer will automatically use a dedicated GPU card and ignore the processor iGPU (and perhaps wish it were just a lump of thermally-conductive metal instead, lol, to help achieve high CPU overclocks).
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