Toronto is fortunate enough to have a lot of options for future GO expansion but I think there are 2 , at least over the next 10 years, that should be given priority:
1} the Bolton line. Even if only for rush-hour service and limited service during the day ala RH, it would prove to be well patronized, would help create TOD along that fast growing corridor, and would be relatively inexpensive to introduce.
2} The Mid-town corridor which would be a real game changer and I think could easily become one of the busiest corridors in the network. It should become a RER type service all the way from Milton to Marham offering commuters a fast way to get across both Toronto and the region without having to go indirectly thru Union. They could still keep the Milton-Union route but only for rush hour service and other times they take RER with a quick transfer to get downtown.
As an added note................ please DON'T come up with another stupid London route. Londoners have suffered enough so please leave them alone.