If GO can't add weekend train service they should split the busiest bus trips into two trips. One that originates at University of Waterloo and runs express via Highway 85, and one that short originates in downtown Kitchener. And the reverse for the busiest westbound trips.
GO already splits many of the weekend route 30 trips into two trips, but only for the following trips:
Eastbound from UW: 09:22, 10:17, 11:17
Westbound from Bramalea: 16:35, 17:35, 18:35, 19:35, 20:35
Excerpt from current eastbound weekend timetable
Excerpt from current westbound weekend timetable
If Brian Doucet was on the 09:17 eastbound bus, then the people left behind by the bus would have been picked up by a second bus 5 minutes later. Or maybe he was on the 08:22 bus which doesn't have a second bus.
Running the second bus as an express would provide a better service to customers by better distributing passengers between the two buses. As it stands now, eastbound passengers will all want to be on the second of the two buses, so they don't need to wait as long for the train at Bramalea. But if there are too many people for that bus, then the people who don't fit are stuck waiting 55 minutes for the next bus, which would connect to the train 60 minutes later.
I think the most important stop for the express bus to skip would be the Syntex & Financial stop in Mississauga, which would save about 5-10 minutes. Depending on the distribution of passengers between the stops in KW, the express could also skip Kitchener GO as you described. The goal would be that the express branch serves roughly half of the trips in the corridor, to divide demand evenly between the two buses.
If UW+WLU account for half of the riders, then the express should just run non-stop from Waterloo to Bramalea.