blogTO's Derek Flack had a nice post imagining the sorts of ways Toronto might develop by 2050, setting down some basic parameters about things needing to be done.

This is a good approach, the only good way to approach futurology sensibly. It's just also important to note that, sometimes, people will not do the things that need to be done. The stagnation dominating Toronto's transit system over the past two decades despite a growing need for efficient mass transit comes to mind. Sometimes, people are less rational than you'd expect, and can derail things efficiently.
TRANSIT
Aside from projects already on the books (e.g. the Spadina extension, Eglinton Crosstown, Finch West LRT, Sheppard East LRT, and whatever replaces the Scarborough RT), Toronto really needs to expand its rapid transit network. The three most likely additions by 2050 are a Downtown/Yonge Relief Line, Waterfront LRT, and some version of John Tory's SmartTrack.
POPULATION
Yes, there will be a lot more people here in 2050. Toronto's population could reach 4 million people, while the GTA will be pushing towards 10 million. It's no joke when urban planners speak of the need to address transit and infrastructure needs now as the trends show almost alarming rates of growth in the absence of appropriate sustainability measures.
Toronto 2050DEVELOPMENT
The city will continue to become more dense, but the most stark visible change will surely be the development of major areas like the Lower Don and Port Lands. 300 metre towers will be common around busy transit hubs, and condos will finally push into Parkdale. The once-quiet stretch of Dupont will be a major residential corridor just as south Etobicoke's population will spike with a new LRT.
COMPLETE STREETS
Those interested in urban planning should keep very close tabs on Eglinton Avenue right now. LRT construction has given rise to the Eglinton Connects project, which is a major effort towards rethinking the design of our streets. Expect our streets to focus more and more on pedestrians and cyclists over the next few decades as development intensifies.
Toronto 2050THE GARDINER
Will this thing still be standing in 2050? I'll go out on a limb and say that a future mayor is smart enough to see the potential in tearing it down and pursuing further development east along the city's waterfront. It will take a bold vision, but should this concrete relic still define our lakefront in 35 years?

This is a good approach, the only good way to approach futurology sensibly. It's just also important to note that, sometimes, people will not do the things that need to be done. The stagnation dominating Toronto's transit system over the past two decades despite a growing need for efficient mass transit comes to mind. Sometimes, people are less rational than you'd expect, and can derail things efficiently.