Of course Apple stores generate the most retail sales per square foot in the USA...selling gobs of small devices priced around $99-850 in a small store will surely get you that title.
As for Amazon, I have no clue how they could bring something new/different/radical/unique to the brick & mortar business model, but I would be interested to see what they come up with, if anything at all.
On second thought, maybe something having to do with same-day shipping? I know they use lockers in staples and are building new warehouses. Maybe they could, at some point, offer consumers yet another way to pick up their stuff. But what do I know xD.