Here is the most recent picture that I believe is the total of what is, at least partially, under that mess.
I think a great idea would be to restore the facades, and add on taller in the rear, similar to what they are doing on two of the buildings on the State St. Project. This would bring back some another piece of historic architecture on 300 S, along with the Brooks Arcade and the Judge building, would proivde some additional retail on the street. I would like to see this is mixed income residential, bringing even more residents downtown.
The great thing about this location is there is a parking garage located on each side east and west, along with one directly across the street. It is very likely that a shared parking agreement could be arrange and no parking on site would need to be accomodated. I do however imagine that at least some spots could be accomodated in the basement of the building.
As for you mentioning the Zims (Pebble and Twigs) building. When I viewed the property about 4 years ago, I was told that the facade was somewhere around 80-90% in tact, underneath that lovely 60's vomit.