MTO is actively planning to add an HOV lane through Mississauga, much like they did in Halton 15 years ago. The Dixie Road interchange project and Hurontario Interchange project done a decade ago or so protect for it, and the Credit River bridge replacement is actually building the lane and will block it off with temporary barriers until the rest of the corridor is built like MTO did at King Road on the 400 for a while.
I would say Halton could also probably squeeze out an extra lane if it needed to, but it would be expensive as it would involve the realignment of a lot of the service roads. MTO has already completed the EA for a widening west of Guelph Line to extend the HOVs through the 403/407 interchange.
Regardless, the QEW corridor already has the best serviced GO line directly adjacent which is a big reason the highway can get away with being only 6 lanes despite it serving a huge population. A lot of the corridor's demand is already using transit. I wouldn't be surprised if weekend trips to Niagara are already at a 30%+ transit modal share given how slammed the GO trains seem to be.