Exactly this.
We can look to MTO's 2051 GGH transportation plan for inspiration:
View attachment 624318
I Imagine we would see:
1. Milton Electrification
2. Toronto Northern GO route - a new commuter rail / metro route from Hamilton to Oshawa via North Oakville, Mississauga, Pearson, Vaughan, Richmond Hill, Markham, Seaton, and Brooklyn
3. GO RER extension to Downtown Hamilton GO
4. GO RER Extension to Kitchener
View attachment 624319
Other potentials which aren't identified in the GGH 2051 TMP:
1. GO Midtown Line
2. Bolton GO line
3. Brantford GO service (connections to Downtown Hamilton?)
4. Niagara RER
5. Uxbridge GO extension
6.Guelph - Cambridge GO
7. GO RER to Bowmanville
8. London GO service