Well, even if Windows could run on ARM architecture, that processor still wouldn't help at all. It's better than most smartphone processors, but still far weaker than the lowest end Intel Core CPU. Not because of frequency, core count, or something like that, but because of the different architectures. ARM is inferior to x86 by its nature.
My suggestion would be that all the CPU power must be inside the phone. Otherwise it wouldn't make sense, because, as I said, if you add a CPU to the dock, it becomes a fully autonomous computer and (1) costs more than the phone (if you want better processing power, that is) and (2) doesn't require the phone to turn on - so what's the point in buying that specific phone then? Oh, and (3) it's more like a computer than a dock
But since, as of today, we don't have good enough mobile processors that would provide enough power and not overheat in a small phone body, I think the only option is to wait and see what Intel brings with 10nm manufacturing process. Maybe we will have similar processing power to today's Skylake-Y in CPUs with 1.5-2W TDP?
Maybe the phone should have a hardware keyboard, but I'm leaning towards the Blackberry approach. To be more specific, small keys that can be used when holding the phone in hand. Of course we don't want to sacrifice the screen size, so the keyboard could slide out, like in Nokia N97.
As for the accessories, I had a thought... why not utilize the Windows 10 touch-friendly interface and make a tablet dock? A tablet with a special slot in the back where you can put in the phone. That way you could have some extra battery and storage in the tablet, and maybe even a GPU (if it fits, because we want it to be lightweight, don't we?) - the tablet would be for portability, or you could have a desktop dock. The buyer could choose to buy either the desktop dock, or the tablet dock, or both of them. Maybe the tablet could also be connectable to the desktop dock to give it some graphics power (you don't need it if the tablet itself has a GPU).
Please tell me what you think but keep in mind that it's meant for a further future project, maybe not by Eve (if they don't have the power required to pull it off), and my opinion is that it's not even worth starting to think about such a phone yet. There is no point with current hardware possibilities.