I would have also said Union Station (ala, Apple Store Grand Central), but it's less realistic - and I can't think of a suitable spot for one. The one in Grand Central is... odd. It doesn't even feel like an Apple Store proper, and to me it's more along the lines of a pop-up store that so happens to be in Grand Central, but it's such a unique experience nonetheless.
Perhaps a location in the Dominion Public Building facing Yonge and Front when the future revitalization occurs? Also, I feel like the foot traffic around Yonge-Eglinton could support another store, the podium at E Condos is a grand space, but... reserved for another TD
- maybe something on the other two corners in the future
... All this being said, Apple has their grounds pretty well covered as far as retail locations are concerned around the city. I think two locations for downtown is sufficient in their minds.