Only grade separated transit needed is the completion of the Sheppard subway line and building the 407 Transitway, both of which are east-west corridors like the 401. Maybe also extending Bloor-Danforth subway along Dundas in Mississauga to connect to the Hurontario LRT, which is another east-west. They are already building the Ontario Line (aka Downtown Relief Line) and Eglinton Crosstown, both east-west lines. The rest can be on-street LRT. Mississauga currently looking into LRT along Derry Road which is another east-west corridor, and very close to the 401.
Future East-West Rapid Transit Corridor (Derry Road)