No, it's not going on the bus station site. The recently published masterplan for Waterdale shows a massive building on the current Council House car park, simply labelled as "SuperStore", linked by an enclosed bridge to a small "Front Of House" entrance more or less where Council House currently is...

www.doncaster.gov.uk/images/ccq-master-sept-0...

If that's not intended to be Ikea I'll eat my hat.