Assuming the St Vital was built with structural supports for a second floor and the floor space would roughly double it would be approximately the fifth largest mall in Canada according to
this list.
It would seem that Winnipeg is due for a mega mall of roughly that size. It was mentioned though parking would be an issue. They are also doing a major update of the existing mall space, something that would seem to make more sense as part of an entire expansion project if it was going to be happening soon.
I think an addition to Polo Park to add more space is more likely.