I believe there is a two-fold issue here. The first being urban planning, Toronto and Canada in general still has a long way to go to solve out horrendous urban planning principles. As well the sleepy-suburbs around Toronto aren't really clamoring to develop anything but more residential. They seem intent on trying the keep the suburban dream alive for as long as possible to the ultimate detriment of residence. The other issue is with Metrolinx and the various transit agencies in the GTA. The fact is the GTA is still very auto-dependent mostly due to our horrific urban planning. Metrolinx and transit agencies for there part don't seem to be to concerned with any of this and Metrolinx further fuels this problem by constructing parking garages of a grand scale. The fact is we know many GO Stations sit on some prime real estate the Metrolinx could sell for some good cash and can be developed, however municipalities may be against this as it brings development they may not want, and Metrolinx also likes the money they make from the parking lots and integration of transit services is still a far off dream.
Basically our Urban planning forces people to drive because public transit outside the urban cores is non-existent. Metrolinx further fuels this car culture by constructing grand palaces for car owners and suburban municipalities don't want any of that "urban" development ruining the quaint little suburbs so they would likely be up in arms if ML started selling off GO properties to developers. As well Metrolinx isn't in the business of Property development/ ownership themselves as that is mostly an East Asian occurrence.
If it were up to me I would create a ranking system for the Stations based off potential for development. It would go something like this:
Class A Stations: Stations that serve as the centre of an urban core. Development of the site should the be the highest priority. These stations should become hubs of transit and commerce ala; Central stations for their municipality. (Examples: Union, Cooksville, Brampton, and Langstaff)
Class B Stations: Stations that are of some importance but don't fit the bill as being a large hub. These stations may be in suburban areas/towns that aren't of the same quality as say Brampton, Mississaugga, or Toronto. Development is encouraged although to a smaller degree.
Class C Stations: Stations that have little to no development potential mostly due to being located in areas of little to no development. (Examples: Lincolnville, Gromley)
Something along these lines is encouraged although the success of it would be based solely on the roles ML and municipalities play. Success could be quite easy to achieve if say ML was able to unilaterally sell GO lands to developers without municipal approval, or if ML was allowed to become a property developer. However if municipalities are allowed to be envolved with the process unless there is an overhaul to our urban planning system, such a development scheme would fall to special interest groups; especially in suburban areas.