Apple wouldn't develop a new chip, the would take an ARM chip. Maybe one with more cores than usually because power consumption is not as important for a desktop / laptop as for a phone / tablet.
Apple wouldn't create any new tools, because the tools are all available. You just use Xcode and tell it to produce code for macOS or for iOS, and for which processor. You can build ARM versions of any macOS app today, except that you won't find a Mac today able to run it.
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It's about two seconds work. In Xcode, don't choose a real device as the target but any of the provided emulators, and the app will run on macOS just fine.
Appreciate the info. So it doesn’t seem like there are as many roadblocks as many would think. Challenges for sure, but I am optimistic.