The whole 403 naming thing is just a mess in my opinion. I just hope that in some future plans it gets sorted out somehow, either by renaming existing sections, or by adding new sections of other highways (the 407 West) to make the naming make sense. Because it's pretty clear that the 403 is never going further east of where it is now, and the 410 is never going further south from where it is now (it was supposed to go all the way down to the QEW).
The 403 naming now is going to be almost as bad as what Highway 7/407 is going to be 10-15 years from now. The highway is basically going to have 3 distinct 4+ lane segments in it, with only one of which actually named 407 (the other 2 being between K-W & Guelph, and Carleton Place & Ottawa).
In terms of the Western GTA though, I think the biggest expressway priority should be completing what would become Highway 406 (I want the name of the current 406 in Welland changed to something else, because that makes no sense at all why it's named 406 when Highway 6 is over 40km away from it). From the Highway 407/406 interchange in Guelph, through an upgraded Hanlon Parkway, through the 401, and then down to the 403. Highway 6 between Hamilton and the 401 is crazy busy, and is only going to get busier.