I know this idea has been thrown around a fair bit, but I'm personally dubious as to the benefits a system of this scale would end up generating, with the sole exception of simplifying cross border boundaries, allowing for smooth travel across urban boundaries rather than the silly cat and mouse games the various towns and agencies engage in right now.
I get the appeal and it definitely makes sense in a place of a certain scale, but I think that the GTA is far too large and sprawling, and with far too many competing needs and interests, to make this a viable course of action or to even result in any real tangible change. The needs of transit in downtown Toronto, or even along a more frequent suburban line in the outer boroughs of Toronto, are very different than the needs of transit out in Milton (though I wouldn't protest to having transit that runs 7 days a week out here!) Outside of a few cross border routes, which could also be solved by improving the frequency of the GO bus and not letting CP Rail hold commuters in Milton and Mississauga hostage, I don't think there would be that many changes to the route network... not like you'd see the 32/34 Eglinton buses extended deep into Mississauga or Milton, the route would become far too long to be run reliably and would necessitate a transfer anyway.
Inter-agency cooperation, and nixing nonsense laws like the ones that only permit the TTC to carry passengers within Toronto, would go a long way towards improving cross border travel in the region without having to spend money on rebranding and what have you.