Well one problem I see immediately is there's no way to machine the channels to run the wiring to the power cable. I also don't like the idea that the FaceTime camera, speakers and mic are located in the slot. And why make the the entire thing generally taller than the original? And the angles are off, the bottom of 128K slants slightly forward, not perpendicular, and certainly not backward slanting.
The simplest fix for all of these things is to make the indent a covered compartment to house the downward facing speakers and provide a place to put the power connector. So it would be similar to buldge on the back of the current iMac, and perhaps part of the machined back panel. As for the balance, the bottom is solid whereas the display is hollow, with very little aluminum. Think the iPhone 6 Plus supported by the new lightning stand. Certainly if the upper part of the Mac is no heavier than the iPad Air (without a battery), the base could easily be heavier to support that kind of weight. And a battery could be put in the lower vertical section, which should be adequate for supplying the whole thing with power for at least 8 hours. The disk slot I've always imagined would be used for USB flash drives and SD cards, in conversions I was planning with original Compact Macs. This may be the best use of the slot, two USB-C, and an SD card slot, and a headphone jack. However, even concealed in the slot is not very attractive, so I'd suggest just getting rid of the slot altogether. Put some convenience ports on the side like a laptop, and the larger ones on the back.
All things considered, this would be very cool. But I just can't see Apple ever doing it, or licensing it to anybody else. What might be more realistic is an iPad Air dock, designed this way.
