My issue with the detachable controllers is that it seems like the wrong thing to detach. If you take the controllers off to plug the main unit into the dock, you can't use the touchscreen anymore. By far the best use of the touchscreen on Wii U was Super Mario Maker. Would you only be able to create stages in portable mode? That would be weird.
What if the SOC and cartridge slot were in a cartridge of sorts itself that you placed in the dock at home or controller when mobile? That way, you could still use the touchscreen if needed when docked. The shell without the main processors would essentially be a Wii U GamePad. Just an idea. That would add some extra weight to the thing though. If this thing is rocking a 6.2 inch screen, it's already going to be pretty unweildy for longer play sessions.
Anyway, I don't think 720p is something Nintendo would have considered if its console and handheld were separate. It's more of a battery drain. It'd be cool if that "brothers in a family of systems" led to a pro/elite version with a 1080p display. Let's go nuts. Throw a glass OLED screen in there and it can retail for 599 US dollars.