I've always thought that a grocery store would work well near a GO station, as they are busiest after work and on weekends when GO parking lots normally have surplus parking spaces. It also seems reasonable that commuters would stop in to buy groceries after getting off their train, before going home. The only issue I can see is that some spaces would need to be reserved for store patrons on weekdays, but I would think there's minimal overlap in peak demand for parking.
Yes, a grocery store was another one of the retail possibilities that I had imagined, although it would take a considerably larger footprint. The Tim Horton's kiosk at Union is literally wrapped around a pole, and doesn't take up very much space at all.
My ultimate vision for the GO parking lot sites is this (assuming a relatively square, large parking lot): 4 parking structures in the 4 quadrants of the site, with an access road on the edge of the site on either side. The ground floor of the structures would be retail, possibly Big Box, but including things like an LCBO and restaurants as well. Floors 2-4 would be parking, while above that would be condominiums, a series of towers at different corners of the structure, and varying in height. The space in between the 4 parking structures would be a pedestrian promenade, with the N-S one leading directly to the GO station at one end and the main road at the other, and the other bisecting the main promenade. The idea is a distinct separation of pedestrian and vehicular flow, as the vehicles are confined to the outside of the site and floors 2-4, while the pedestrian flow is on the inside of the site in an urban village-like setting.
In general, that would be a much more efficient use of the parking lot space, and would actually create a TOD community around each station. Not to mention, it would be a good way to disperse the evening rush hour parking crowd, because they could stop and pick up dinner/do groceries/pick up a bottle of wine/actually stay there for dinner, instead of just rushing to their car to leave.
And smallspy, I didn't know the Oakville parking garage was going to include retail. It's really too bad they didn't, although with Oakville at least there's retail within the parking lot complex (Taco Bell and a couple others I believe). Stations like Burlington only have a Wal Mart that's across a field adjacent to the station.