We will definitely see something more advance than the A12Z, considering the mac form factor have much higher headroom for thermal and physical space.If you have been listing to Craig F. and company over the past few days, you have some clues. First there will not be a mac with the A12Z, they are doing a Mac only processor. The A12Z is 8 cores, the new Mac processor is rumored to be 12. I expect that the 2020 systems will be a Mac Mini and a MacBook Air. Since they are using the Mini as the developer system, just update the processor and go. They are going to want a low-cost laptop like the MacBook Air to show the power and the battery life you can get from Apple Silicon. 2021 will see an iMac and the MacBook Pro, leaving the Mac Pro for last.
All of these systems will be priced as they are now.
Looking back at the intel transition, Apple used the iMac and the Macbook Pro as their first intel models. Definitely to show their confidence in the new architecture. I would definitely expect the iMac being one of the first lineup receiving the ARM treatment.