I think one of the main issues with getting space like the former Guess store, and Future Shop leased right away has nothing to do with Toronto, but the retail market in general. Landlords everywhere are struggling right now. A lot of the spaces left behind are either not suitable for those retailers currently looking for space, or too large for smaller retailers that are looking for space. Some can be subdivided, but a space like Future Shop at 10 Dundas is already awkward to begin with. Subdividing it would make for even more awkward spaces.
It would be ideal to have Future Shop's space subdivided in two, with a hallway leading from the Yonge Entrance to the escalators within the core of the building. The retail could open up onto this hallway, or into the existing core. This would create another main entrance to the mall from Yonge Street.