All of the pocket parks springing up across the downtown are nice, but the lack of a single large multipurpose park in, or adjacent to, downtown Toronto is probably the biggest strike against the city--at least insofar as it's overall layout is concerned.
A rail deck park connected to the Fort York green space could provide a decent size urban space, albeit, a fairly narrow, fractured and heavily shadowed one.
The last great hope--if it ever materializes--for a great sprawling urban green space, walkable from downtown, is the current city island airport. When combined with the current park space of the islands it would provide around 700-800 acres of space, reachable by foot or ferry. (It must have generous land connections (tunnels, bridges or both) over both Eastern and Western gaps to count as an accessible, legitimate downtown park--as opposed to being an excursion.) That size would be in line with some other great urban parks like Central/Stanley/Mont Royal, etc...
But, if that eventuality never occurs, then we're kinda SOL in that department. And that's a bummer!