That plaza has such great potential. It's owned by Kanbar, so you never know with them what you will get. I can't see them wanting to spend the money on building a new garage themselves, but I could see them selling off the plaza to someone else to build one.
There were plans a while back to turn the plaza into a restaurant area, that all fell through - still think it would be high value doing something like that. A much smaller scale of Rockefeller Centers plaza area with a sunken restaurant and rebuilt plaza area could really change the feel of that area of downtown. Add some infill on the parking lots next to the plaza and add more seating areas for restaurants on the ground floor and Main Street would be reborn.
There's a sunken plaza in Chicago where the plaza is the outdoor patio for a Cheese Cake Factory location, I think it's Water Tower Place. It's a very cool space that could be easily replaced with a local restaurant.