[BIMBAM]

You should definitely do this. When we built the metro in Montreal, we used the earth to build Ile Ste. Helene where we hosted Expo 67. It's huge, home to an amusement park, festival grounds a metro station and extensive parks. It worked out great for us and in land hungry Toronto I bet this would work even better. I think it would be a better sell, however, to build it as one big island, build a bridge, and give it a human use rather than let it go to nature. Better chances of it getting funding, and in such a prime location I'm sure the people of Toronto could find a very unique purpose for it, possibly even an iconic one that could help define the city in the way that liberty island's strategic location and use defines New York and its skyline.

Check out Ile Ste. Helene on google maps, it'll amaze you knowing that it didn't exist just a few decades ago. Perhaps this theoretical island could even be expanded with further subway excavations to produce even more valuable lake front property. With PPP perhaps this idea could even help fund the transit expansions themselves.