As RER begins to offer bus comparable frequencies in the 905, many more people will opt for it as opposed to bus-to-bus transfers from one city to the next. This means the first, and most important step, is fare integration with GO/RER and local systems. Transit system transfer from one city to another will be complicated but system to GO is pretty easy and could be implemented within a few months. All they have to do is offer 100% discount on GO fares from the fare you just bought to get to the station.
So taking Weston as an example. Currently taking the bus to get to the station and going to Union costs $5 plus TTC but if fare integration was complete it would be $5 minus TTC. All of a sudden the fare from the Weston area drops from $8 to $3.....a big savings. Of course the travelling public would love it but it is also very politically palatable as it wouldn't cost the local systems like the TTC a cent as the GO fares would stay exactly the same. The 905 systems would also like it as it not only wouldn't cost them a cent but would probably increase ridership as their trip to Union is made significantly cheaper resulting in fewer people parking or going Kiss n Ride and more taking transit to get to their nearest GO station.