Rather than A12, Apple would be more likely to use the A12X. I suppose the might go crazy and wait until fall 2020 and use the A14X that’s expected for the 2020 iPad Pro. It would make for an awesome gaming platform.
I’d be very surprised if iMac Pro isn’t updated next year, WWDC is a likely venue. Intel has announced the CPUs, though it’s unclear whether they are shipping in volume yet. The Mac mini should also see at least a CPU refresh.
The 13” MacBook Pro could be replaced with a 14” model, my guess is it’s more likely than not. It would provide some additional differentiation from the MacBook Air, which should see a CPU refresh in 2020 as well.
If there is a MacOS-on-ARM product in the lineup next year, it might come in the form of a reintroduction of the 12” MacBook. It would be nice to have a 2 lb. model in the lineup again, 1.75 lb. would be amazing.
Things I or others would like to see but probably won’t happen due to insufficient demand: SE-sized iPhone; iPad mini Pro, Mac mini Pro (larger enclosure with a PCIe slot for GPU); Mac Pro mini (the fabled xMac/mid-tower without Xeon/ECC); 27” 5K monitor based on the iMac chassis.
Did you mean A13? iPad Pro already has an A12X chip.