I would guess any ARM MacBook 12" would be introduced sometime in 2020 at the earliest, but it would be fun to be proven wrong. Reasonable updates could be very slightly faster CPU (no big deal) and/or possibly other hardware upgrades such as better face time camera, 3rd gen keyboard and a second USB-C port in place of the headphone jack. Here's one update that would be nicer than anything I've mentioned above: exact same model with a price cut!