As pointed out by John Gruber, the transition would be very difficult.
The mythical Apple hybrid OS
Other rumors point to this direction. Apple is expected at some point to build that mythical system mixing the benefits of iOS and MacOS on the same device. More specifically, Apple is said to be allowing iPad Apps to run on MacOS. This is an old rumor, that I could even trace back to 2010, with John Gruber, Dave Winer and Rene Ritchie debating how this could happen.
Recently debating on the topic, Paul Haddad ironically said he was expecting Apple to let iPad app be submitted to the Mac App Store. Such app could look as rough as an iPad app running unmodified in a window like the simulator used to test iPad applications from Mac during development, very much like Web App are packaged as a desktop app with Electron. Jean-Louis Gassée, former Apple executive, also wrote on the transition, using the iOS simulator as a reference.
A better CPU transition scenario
This is not something that would be user-friendly and energy friendly. I do not think this is the path Apple would take.
There is a better way forward, first hinted by the current architecture of the MacBook Pro with Apple Touch Bar. The Touch Bar is actually autonomous, powered by an Apple T1 chip running WatchOS. The Touch Bar is a bit like a mini Apple Watch.
Then, there is the new iMac Pro, that was first expected to ship with an Apple A10 chip, but actually is using a new Apple T2 processor.
At this point, you should already guess where I am heading to. The transition from Intel to Apple CPU in Macbook / MacBook Pro and the ability to run iPad apps on MacOS would be so much easier if Apple new laptop could rely both on an Apple and Intel chips.
If Apple was to include a new Apple CPU, along with an Intel CPU, inside their Macbook, it would make a lot of sense. You could make it possible for developers to target the new Apple chip to run new apps. Users could run unmodified iPad directly compiled for Apple CPU architecture. Developers could develop for iPhone and iPad, enjoying a simulator, that is just a window of an application running on the Apple chip. Given how energy efficient are Apple chips, it could even be a way to significantly improve MacBook battery life, by offloading the less demanding tasks to Apple CPU.
An interesting move over several years
Will it happen? Is Apple ready for such a technical challenge? Well, there is still a lot of technical issues to solve to make it possible, and I have no idea Apple wants to address the challenge of running an OS that can rely on two different types of CPUs. Laptops are already able to rely on different GPU (Graphical Processor Unit) depending on the type of tasks, but for CPU it is still pretty much a new area.
Apple is known for its ability to make hardware transitions as smooth as possible, like the move they did from PowerPC to Intel processors. I do not know if this is the path it will take, but no matter how Apple does it, it will be a very exciting challenge to witness.
If the change is going to happen, I do not expect Apple to announce it this year at its developer conference (WWDC). What they could announce this soon is an updated SDK to allow building apps with a User Interface that will be rendered slightly differently on iOS and MacOs, to offer the best user experience given the context. It will help developers plan ahead, while still not revealing yet the big picture before the actual hardware is ready.
Before that, I am looking forward meeting some of you in early June at Apple WWDC 2018 in San Jose!
I rarely write about rumors and speculations, but that transition scenario was so interesting that I thought it was worth sharing. I hope you also liked the idea.