Bike lanes, pedestrian usage, will be a problem at the freeway intechange, unless the 403 interchange is significantly altered. If you don't know why this is, then you have never been a pedestrian or a cyclist at such a highway interchange. Ideally there should be a street which has no interchange to the 403 that will be used for local traffic, pedestrians, and cyclists.
You yourself have endorsed the building of an alternate street to the west of Hurontario to be used as a main street.
The way Hurontario is used now is primarily as a way to get from one freeway to another. or at the very least, as a quick way to get onto a freeway. I'm sure you understand why this conflicts with any grand scheme to make it into a multi-modal urban corridor.
You do know that the Hurontario/403 interchange is not in Cooksville, right? Yonge gets kind of dicey near Lakeshore but the Hurontario/403/Rathburn interchange is farther from Burnhamthorpe - let alone Cooksville - than Yonge & Lakeshore is from Yonge & King. One doesn't really have any effect on the other.
Actually, I endorsed ripping the roof off Square One (albeit, with more colourful language) instead of trying to build a cute little retail street immediately next to Square One - which is what Mississauga is thinking of doing. Stores are necessary, an enclosed mall isn't. Don Mills Centre hasn't turned out that great but the intentions are good and other malls/developers/planners can learn a lot about what to do or not do. Square One would be a very different project, anyway, being larger and having two storeys.
Improve what you have...it's guaranteed to be better than starting from scratch. Why endorse starting from scratch when you think it can't work? Why would starting from scratch work here when it's failed pretty much everywhere else and might still fail next to Square One? The off-ramps on Hurontario near the 403 are not intended to be a main street any more than the ramps to the 401 on Yonge are intended to be downtown North York or the Gardiner ramps are intended to be the focus of Toronto. Who cares about the 403 interchange? It's not even that bad...real people aren't so spleeny that they cease to function when they stray more than 100m from quaintness.
Hurontario between Marilyn and Cooksville, though...there's some real potential. Toronto's towers in the park are already being redeveloped and Mississauga's will, too, eventually. An urban area as large as central Mississauga can handle multiple 'main streets,' anyway, just as Toronto has Yonge, Queen, King, Bloor, University, etc.
No, Hurontario is not used primarily to get from one freeway to another. Maybe, just maybe, the many stores, jobs and people living along Hurontario generate some trips every day.