An environmental assessment & an expansion plan study is already underway for expansion along the Niagara Falls route for rush hour monday-friday train service. However, I can tell you that they are EXTREMELY keen on expanding ALL DAY service to Hamilton ASAP. Its next up on the priority(not just in terms of importance but of what can be done sooner) after the Kitchener Rail Expansion.
Management(the contractor - operations side of the service) is already preparing for it(by preparing I mean personal wise, since it takes YEARS of training to become an engineer of which we have a bit of a shortage of), they wouldn't be doing so unless GO told them to.
Expansion plans;
Lakeshore East to Bowmanville - high priority, mid term
Stouffville to Uxbridge - low priority, long term
Stouffville service expansion - mid priority, short term
Seaton line - low priority, long term
Locust Hill line - low priority, long term
Peterborough - no current plans
Richmond Hill expansion - high priority, mid term
Richmond Hill service expansion - low priority, long term
Barrie expansion (one station) - high priority, short term
Barrie service expansion - mid priority, short term
Bolton line - mid priority, long term
ARL link - highest priority, mid term
Georgetown to Kitchener - high priority, short term
Georgetown service expansion - high priority, mid term
Orangeville line - no current plans
Milton service expansion - high priority, mid term
Lakeshore West expansion to Hamilton - high priority, short term
Lakeshore service expansion - high priority, short term
Niagara line mon-fri service - high priority, mid term
where as short term is within the next 3 years
mid term in 3 to 6 years
and long term 6+ years