Several Mac computers running macOS 10.13 have been remotely identified through recent server logs across publications while the first public instance of macOS 10.13 was spotted on Mac App Store, according to a recent report by Pike's Universum. The report suggests that a secure HTTPS link originating from Apple's servers has been identified within a small portion of Mac App Store's Catalog URL.
The Catalog URL (below) and its https format confirm that Apple is poised to bring out the next public iteration of macOS at the upcoming WWDC event, as it seems to be working on the developer preview version of macOS 10.13. https://swscan.apple.com/content/catalogs/others/index-10.13seed.merged-1.sucatalog.gz
Although there are no details available about the content of the next macOS update (v10.13), it is safe to assume that it will be a minor upgrade over the forthcoming Sierra 10.12.4 update which will reintroduce iOS 9's Night Shift mode on Macs along with Siri enhancements.
Some of the noteworthy features added with the macOS 10.12 update include notification and keyboard improvements, Desktop and Documents Sync, Universal Clipboard and Auto Unlock.
Apple is likely to announce the next major upgrades to macOS, iOS, watchOS and tvOS at the annual developers' conference which is set to kick off on 5 June.