I'm not sure if 64GB is happening, it kind of adds unnecessarily to the cost, but still isn't enough storage to really satisfy those people who like to download a lot. I think they're going to ditch the 2 SKU set up entirely.
What I could see happening eventually is perhaps an official Nintendo Wii U HDD or USB Stick that expands the storage more significantly and is plug n' play ready, because I know some people get confused as to which HDD they have to buy for the Wii U.
A few months ago I had to actually help out a parent who was trying to get a Wii U HDD because the Best Buy floor rep didn't know what he was talking about either (he was about to sell the parent a standard 2.5-inch external HDD, which doesn't work on Wii U without an X/Y cable).