I don't think Apple is going to start off with some power device for developers.
Not as an actual product - but it would make sense to produce a limited number of registered-developers-only lashups to help bootstrap the availability of compatible apps between WWDC and when the "real" product comes out.
How will developers test if they debut new tools (Xcode updates, etc.) for the ARM migration, but no hardware?
That's why there may be a not-available-to-the-general-public developers' system at WWDC. Or, in this day and age, it might be possible to do it all with software emulation and code reviewing tools on an x86 Mac. ARM apps might run slowly on a software-emulated ARM but it's enough to test them. The actual compiling can be done on an x86, just as for iOS apps at the moment. Anyway, it's not going to be
about writing unique code that only works on ARM - it's about replacing any specific x86 features with calls to the proper Apple OS frameworks (Metal etc.) and leaving the optimisation to the compiler. It's 2020 and there shouldn't be any place for CPU-specific features in
application code.
What happens to Thunderbolt in all this? I could be wrong, but last I read there are 0 ARM-based Thunderbolt-certified devices, and I believe only two non-Intel Thunderbolt-certified motherboards for AMD. If Apple releases an ARM-based Mac with Thunderbolt, I think it would be the first major OEM to release a non-Intel Thunderbolt-certified device.
I don't think it's that big an issue: AMD boards with Thunderbolt
do exist and a lot of the scarcity is probably down to low demand for Thunderbolt in the Windows world. They can license the "Thunderbolt" name from Intel if they want, or just badge it as USB4.
If the switch to ARM is inevitable, so what's the point of having MacOS or even the Mac line up if it the same as hardware the iPhone and iPads use? So what becomes the differencing factor?
First, the biggest differences between using a Mac and an iPad or iPhone are
already the operating system and user interface (mice/trackpads/keyboards vs touch-screen). Those were Apple's design choices when they designed iOS and nothing to do with ARM vs x86. Apple
could have chosen an x86 when they made the iPhone and the only thing users would have noticed was it was twice as heavy and the battery life sucked. The design decisions would have been the same. iOS already runs on x86 by the way - that's how XCode lets you test iPhone software on a Mac - but using it without a touchscreen is a drag.
Potentially, there's no reason that a user - or even a developer (beyond the small fraction of development work that needs direct reference to the hardware) should notice the difference between MacOS on ARM and MacOS on x86 - just as you couldn't tell the difference between 68k and PPC or PPC and Intel other than by comparing speed. The difference between iOS and MacOS, though is pretty stark...
Second: the power of iPhones and iPads is severely limited by battery life and heat dissipation. Macs have space for bigger heatsinks, fans and bigger batteries so - potentially - ARM processors in Macs will run faster than in tablets. Pro models will be able to offer things like discreet GPUs and Thunderbolt (or USB4).
However - it
is slightly odd that Apple have also been working on iPadOS and the iPad Pro to remove some of the limitations mentioned above, adding support for keyboards/mice/trackpads, improving iPad multitasking, adding a
sort of Finder in the form of "Files" etc. meaning that the iPad and lower-end MacBooks are already in contention. There's a slim possibility that the new "Macs" could actually be "iPad Laptops" running iPadOS - but that would throw away what is
already the key difference between iPad and Mac and ensure that products that
could be received as "better iPads" got bad reviews as "knobbled Macs"....