Personally, if Toronto did get the Olympics, I'd like to see Downsview be used as the site for the Olympic Park. Have the Feds pay to move Bombardier someplace else (perhaps the new Pickering Airport?), and redevelop the site as a full Olympic Park.
Would be perfectly connected to transit (probably the most connected area you could build it), with 2 subway stations and a GO station around various parts of the site. It would also be a good catalyst for a Sheppard West subway extension.
There's enough room for a ~75K stadium, a good sized aquatic centre, and a few other smaller venues. The stadium could be combined with an NFL franchise bid, as was mentioned earlier in the thread.
Also, re: downgradeable stadiums: The stadium built for the 96 Atlanta games was specifically built with conversion to baseball in mind. You can clearly see the baseball setup "imbedded" in the Olympic stadium setup. Heck, you can even see where the dugouts would eventually be.
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