BMO is a soccer stadium. It would be ruined if it had to cater to both the Argos and TFC since the field dimensions are different. If the Argos moved in then there goes the soccer atmosphere that has made it the top soccer stadium in North America
As for the World Cup ever coming to Canada....
Well 2014 is in Brazil, so we won't be hosting anything for that one. 2018 and 2022 are still up for grabs but England is probably the favourite for one of them.
Canada will never host a World Cup (at least not in my lifetime). Maybe a joint bid with the US would be the closest thing we could get, but I don't know why they would want any part of us considering the CSA is in shambles and we don't have the infrastructure they do. They'd make far more money keeping the tournament in the US. In 94, they used football stadiums to host games, with RFK Stadium being the smallest at 57,000. We have 4 stadiums in this country that are 50k+ and one is the dilapidated Olympic Stadium in Montreal, and there are 5 others that can be expanded to RFK's size (roughly).
However, just because they have the seating requirements doesn't make them ideal for hosting an event like this. When you compare the quality, Canada cannot compete with just about any country that wants to make a serious bid without having to build a couple brand new state of the art stadiums (at least 70k each) and then spending billions on upgrades on most of the already existing stadiums. These would be temporary improvements since the CFL can't fill stadiums of this size. So essentially you're building 2 new stadiums, and upgrading 6 others so they can each host 5 or 6 games. Where would this money come from? The CSA can barely afford to survive, never mind invest in world class stadiums.
Let's say we did bid and won. What happens when 40,000 fans converge on Regina? do they have the infrastructure to cope? no, they don't. How will fans follow their teams from city to city? By then we have no clue what the airline industry will look like in this country, and you can forget about trains. Even if the airlines are healthy, they can't cope with the sheer volume of numbers that need to be transported.
Of course, one could look at 2010 in South Africa and 2014 in Brazil as examples that show anyone can get a world cup, but both countries only got it because FIFA wanted a continental rotation. Brazil didn't even compete against another country in bidding. Now that the rotation is complete and FIFA has gone back to allowing bids from anywhere at anytime, its more likely that Europe will host every other Cup (thereby limiting our chances once again) and a Canadian bid would have some stiff competition, and I just don't see how a committee could look past all of the above.