Cookesville GO is on the Milton Line, which is not planned to have RER service, unfortunately.
While the Milton GO line is one of the highest ridership GO lines, Metrolinx does not own the corridor, which limits what it can do. GO RER as planned today is primarily on corridors Metrolinx owns.
The Milton Line is owned by CP Rail, which does not permit off peak service on the line in order to prevent it interfering with their freight movements. As it is today they only permit service during narrow peak periods, where Metrolinx runs as many trains as it possibly can to meet demand.
Metrolinx has potential long term plans to add new dedicated tracks along the corridor to allow for 15 minute service, but this is heavily dependant on CP agreeing to the plan. Which again, is totally up to CP. No funding for the plan has been identified at this time and it should be considered a longe range plan.