I don't know if it's plausible for Metrolinx to get the shovels in the ground today. But I would like to see an accounting of parking at each station, and a plan for future use to, say, the 2050s.
The plans should include timeframes for developing, or not developing, Metrolinx-owned lots near GO stations, as well as a commitment to improve the pedestrian and cycling access in the immediate approaches at all stations. For example, here's what I drew up for Rouge Hill GO in about 2 minutes:
View attachment 578735
I would like to see this exercise (with slightly more detail) repeated at all current and future GO stations, minus the few that don't have park and rides.
I've been thinking along the same lines as you, though my thinking is that we can redirect parking demand by having different rates for different stations. Stations in urban growth centres like downtown Brampton or downtown Kitchener should have very expensive parking (if any at all) while stations with less development potential like Bramalea or Breslau should have cheap/free parking. So the question becomes just "which lots have the most developmet potential?".
For Rouge Hill I actually prioritized the north lot for redevelopment since it's less valuable as parking than the south lots and more valuable as residential. People generally prefer to live not directly next to the station but a short distance away where it's quieter (both in terms of activity as well as literal noise). The south lots would be a great site for a commercial/office development but there's no market for that at the moment.
My sketch for the north lot includes 41 rowhouses, a neighbourhood parkette, and a mixed-use building with a midrise podium and modest towers above.
And without further ado, here are my subjective top 5 GO Transit parking lots to redevelop ASAP:
1. Brampton Central - 0.86 hectares. What on earth is a park-and-ride lot doing in downtown Brampton?! Just drive to Bramalea!
I've highlighted the south lot because it's closer to downtown but the north lot should be redeveloped as well. I remember hearing plans for redevelopment but not sure what the status is at the moment.
2. Rouge Hill - 1.63 hectares, as described above.
3. Rutherford - 0.82 hectares. No need for this surface lot now that the garage is open. Could go highrise here.
4. Kitchener Central - 0.25 hectares. This site is already offered for development as part of the Kitchener Central station plan but the Region hasn't been able to find an interested developer so it's going to be a parking lot until they can find one.
5. Maple - 1.47 hectares. Just copy/paste more of the same development pattern. This parking lot is very wel used but we can shift some demand to Rutherford, cycling and transit. Once the new bus loop opens (already complete as you can see below) YRT could run more convenient connecting bus service. There are also plans for a bicycle/pedestrian bridge across Major Mac to connect the station to the neighbourhood to the south, making cycling much more attractive. The original plans for the station reconstruction also included a new entrance on the west side of the tracks to allow those residents to walk to the station, but residents on Railway Street freaked out about having a railway station on their street so the entrance got removed from the plans and people from that neighbourhood will need to keep driving around the block to get to the other side of the tracks.