I really like that platform layout. Assuming these will be the Lakeshore platforms, it makes it really easy to have the island platform be for the Lakeshore trains. Through-running trains can use the portion that continues west or east, while the short-turning trains (be they express, or special event, or whatever) can use the dead-end tracks on the same platform.
Of course, there's no reason why this setup needs to be exclusive to Lakeshore. It could just as easily be a Kitchener-Stouffville pairing. The dead-end tracks could also be used to balance out any differences in frequency between the two branches.
Whatever the assignment, I think it would be beneficial from a customer wayfinding and usability perspective if each island platform that was created (assuming the platform reconfiguration within the train shed itself uses the same setup) was assigned to a specific pair of routes. That way permanent signage could be installed in the concourse, and you would have less congregating in the concourse waiting for the platform number to pop up on the screen.