I don't know where on earth you'd find a parcel of land big enough downtown to support a big-box grocer like Superstore or Walmart unless it was part of a larger development like in the podium of Skycity.
To put something the size of the Taylor Ave Walmart (store + parking lot) anywhere near The Forks you'd
pretty much need to demolish Shaw Park and reroute streets. If you cut down the parking I guess you could eliminate the street rerouting part.