Updated, 3/12/2018, 2:00pm PT: More clues about the Z390 chipset's potentially imminent launch arrived via AIDA64, a "system information, diagnostics, and benchmarking solution" regularly used to test pre-release hardware. The software was updated to add "motherboard specific sensor info for MSI B360/H310/H370/Z390-Series." That could mean that MSI is preparing to launch a board equipped with the Z390 chipset.
Original article, 9/19/2017, 3:40pm PT:
We are obviously on the cusp of Intel's Coffee Lake release, and we already know the processors are coming with Socket 1151 (the same as the existing Kaby Lake era processors) but will require a new 300-series chipset. However, if the latest leaks/rumors are to be believed, the initial wave of Z370 motherboards will be followed shortly by yet another new chipset.
A leaked Intel roadmap claiming a Z390 chipset will come in the second half of next year recently made news, but as with all leaks, we have to take the information with a shovel full of salt. However, a purported Eurocom representative claimed the company is readying products that support a new chipset: Z390. Eurocom is a boutique laptop manufacturer that specializes in high-performance products.
The post on the NotebookReview forums claims that the company is skipping the forthcoming Z370 in favor of the Z390 chipset. The post also claimed that the Z390 motherboards support 8C/16T processors and will arrive in the second half of 2018.
Whether or not the claimed eight-core models would utilize Intel's 10nm process, which is slated to appear with the company's Ice Lake processors, is an open question. Cramming in two more cores with Coffee Lake is quite the feat with 14nm technology, be it 14nm+ or 14nm++, but doubling core counts with the same process is unlikely due to thermal and power constraints. That implies the eight-core processors will feature Ice Lake's 10nm+ process, but the leaked roadmap specifically refers to the Z390 chipset for "Coffee Lake-S" processors.
Intel's 8th Generation lineup includes the 14nm+ Kaby Lake-R (refresh), 14nm++ Coffee Lake, and 10nm Cannon Lake processors, which might explain the need for another new chipset. Intel's disclosures indicate the company is moving forward with a new rollout strategy as the competition intensifies, so it will likely be some time until we have the complete picture of the company's plans. We can expect more info to come forward during the pending Coffee Lake launch.