Well, you could probably abandon hopes for a 'supertall' being proposed within the next decade or so. In the longer term though, I see no reason why a supertall is unlikely. I doubt there is any way commercial development will move to a 'European' model of mid-rise buildings spread over the downtown core. Zoning regulations are completely stacked against this. For right or wrong we all know development in the low rise portions of downtown is the difficult. European businesses hate this anyways and there is no economic sense to it, just look at the success of Canary Wharf relative to the City and similar cases in Paris and other markets. A more likely scenario is a greater move to suburban office parks. That is probably the most likely for any business that doesn't derive a tangible benefit from location downtown.
So, if we ever do get a commercial supertall, it will almost definitely have to be finance related. Under current financial regulations and market spasms though, no banks will be looking for a big block of office space though. Going forward the Canadian Banking Act will have to be revamped. The only way a big enough demand for office space could develop is if foreign banks were allowed into the Canadian market. Canary Wharf basically works as the European HQs for American Banks like Citi Group, Morgan Stanley, BoA, Baer Stearns and (formerly) Lehman Bros and all the services these use. Toronto would have to attract these companies in a big way, which is currently impossible.
EDIT: Another way could be government related. I don't have any statistics on this, but I imagine if you add up government office space maintained by the 3 levels of government in Toronto you could easily fill a 'supertall'. Basically sell of all government offices not in Queen's Park or New City Hall and move them into one supertall. It could potentially be justified on cost savings from maintaining one central facility.