The goals that I see are

1. Be highly energy efficient
2. Replace the many custom power warts with one generic power cord

I think that we will be stuck with 100-240V AC distribution within the neighborhood and within the home. This meets the needs of the many high power devices and is the installed base. I had relatives who participated in the conversion of Boston from DC to AC. Given the immense work that it took then I cannot imagine replacing the current AC distribution systems. The need would have to be earth shattering.

For low power devices (under 10W and under 10V) I do expect the outcome to be a split power supply. The generic DC cord will provide up to 1A of variable DC. Then each device will take the variable DC and internally provide regulated power at the needed voltages.

This is much more energy efficient than any low voltage distribution will be. The generic DC cord can be made efficient, although the combination of generic DC with custom regulation is probably not a match for the best switching power supplies. But much of the waste in the low power devices is leakage waste, and that could be made quite low.

I have three devices that are close to this:

1. My antique Psion 5, whose custom power adapter is about the size of a 3prong plug.
2. My Zaurus CL3000, which has a really tiny little power adapter. It's 4x4x1.5 cm. It provides 1A at 5V nicely regulated. The size is a bit misleading because the prongs are pivot mounted and can fold into the supply. When folded, the whole thing is just 4x4x1.5
3. A mystery adapter. I wish it were properly labeled. 1x3.5x1.5 inches, producing variable DC (5-10V) and accepting 45-65Hz 100-240V.

A standard 3-prong plug is 2x1x1 inches. All of these exclude the actual prongs. So current products are close to meeting the size goal. I expect that the generic 10W DC cord will emerge. It can be introduced fairly easily because on one end it can use the existing 45-65Hz 100-240V power grid, and the other can be a generic 10W DC.

You could push the power rating up further but when you leave the domain of pure electronic devices and introduce motors you gain some difficult power supply issues. There are many reasons for the custom voltages when driving motors, and usually the cost tradeoff between custom voltages and difficult mechanical design ends up with custom voltages.