I've heard that it may close off 2nd and 3rd between San Jacinto and Trinity, though Trinity will remain open. I can't confirm that, though . . . just what I was remembering.
The last convo I had with people at Visit Austin (in Jan), they were still pushing the idea that it will have lots of ground floor retail and a public park on the roof (see below). Not sure how that would work practically speaking, but there aren't other CC's with that feature, and it's viewed as a way to activate as much of the CC as possible with additional non-CC uses.
The preliminary plans, so far as I know, don't include housing.