To add to
Bigtime's post above, I've made up another image with some potential sites.
The River Pathway shadowing bylaw is already set in stone and is not going to change. I've also pointed out some other parks/plazas in downtown which do not currently have shadowing bylaws, but which under the Centre City Plan (which is now the basis for the direction of planning in the city core) were listed as being potentially protected by similar, if lower level, bylaws.
The most obvious site for a supertall currently is the one Bigtime pointed out, which Imperial Oil currently owns. The next ones would be just north of the CPR tracks, and then after that just south of the CPR tracks.
Of course, these are just sites where it might be allowable and are right now covered by surface parking. If someone is going to build a really tall tower, there are older and smaller buildings that might be worth knocking down.
And that's all besides the economics and the big "who would occupy this tower?" question.