There were detailed plans for a station at Breslau, potential stations on both sides of Guelph (Watson Parkway was indeed studied) and station and yard on the west side of Kitchener. All of these would include GO's sea of parking.
Acton was also included in those plans, its opening this week is likely the first step towards adding additional stations, especially now that track and signal upgrades were finally given the green light. There's also work around Silver Junction (west of Georgetown) right now.
Glad to see that my planning intuition isn't completely off
If that many stations are in fact built, it may make sense for GO to run trains from Ira Needles (assuming that's where the station is likely to be, or perhaps Fischer Hallman) to Watson Parkway, as a supplemental service to the interregional GO service. Perhaps an hourly all-day service? I would think a service like that wouldn't require substantial double tracking.