There has been more than one leak suggesting that Intel is getting ready to launch an 8-core Coffee Lake processor, though the strongest evidence to date is a series of technical document listings on the company's website.
A user on Reddit discovered the listings, of which there are three that reference an unreleased "Coffee Lake S 8+2" part. Unless Intel suddenly changed its nomenclature, the 8+2 designation points to there being 8 CPU cores and integrated GT2 graphics on this particular chip. For reference, CPUs like the Core i7-8700K are listed as being Coffee Lake S 6+2 parts, indicating 6 CPU cores and GT2 graphics.
Unfortunately the documents themselves sit behind a login page, presumably for registered developers, so we can't click through and view them. However, since the listings appear under Intel's "8th Generation Technical Library," the assumption is that the 8-core part(s) will be marketed and sold under that generation. We can further extrapolate that they will be compatible with existing 300-series motherboards for Coffee Lake, if that's the case.
None of this is confirmed, however, and with Intel having introduced a new Z370 chipset that is required for current Coffee Lake chips, we wouldn't rule out Intel doing something similar with its upcoming 8-core parts. In fact, some of the speculation that is out there is that Intel will release a new Z390 chipset for a 9th generation Core processor lineup that includes 8-core CPUs. Intel's new H370, B360, and H310 chipsets also added a couple of new features that Z370 doesn't support, like 'modern standby' with wake on voice support, so Z390 might simply be the superset of all 300-series chipsets.
In short, we have more questions than answers, though we're optimistic that Intel will make these upcoming chips compatible with existing 300-series boards. Hopefully Intel will answer our questions soon.