This is a very active conversation in soccer circles but runs into two issues:
1) CMNT/CWNT don't really need a large-scale venue of that size. BMO is right-sized and BC Place right-sized for larger crowds.
2) The usual anti-Toronto rhetoric from everyone else about how a "national stadium" shouldn't automatically be in the GTA.
Frankly, the only way we get a 50-60K stadium built in Toronto is if it's unfortunately tied to an NFL project. I like to believe that Rogers Stadium is a test for a future NFL stadium on that site but i'm not holding my breath that we'll see any movement in the near future. Slowly expanding and building BMO out might be our best option, but realistically CMNT/CWNT don't need a stadium larger than 35-40K in any scenario either short-or-long term.