Well, first of all, Hurontario LRT has been approved, and it is only a matter of time before it gets built. Even on a Fantasy map, I am reluctant to place the subway and the LRT in the same corridor
If the B-D subway ever gets extended to Square One, it will almost certainly take a more direct route.
Secondly, I am not very enthusiastic about bringing the B-D line to Square One, as for all the cost, it will not create a truly fast link between the Mississauga's and Toronto's downtowns. The trip from Square one to St George, with a transfer to Line 1 and a ride on Line 1, will take close to 1 hour.
If there should be a heavy rail link to Square One, it should be either a branch of the Milton GO line, or a completely new subway with a wide stop spacing. In that case, we can hope to reduce the trip time between the Mississauga's and Toronto's downtowns to something like 25 to 35 min.