Aren't there utilities, etc in the way of your proposed pedestrian connection between Union and ... Union East?
I know it's an unrelated project. However I had suggested extending the Lower Bay Concourse under Bay st to connect with the Bay East Teamway as a way of eliminating some of the jay walking that happens on Bay. I recall the buried utilities posed a barrier to this suggestion.
It wouldn't be a Union East, rather a single interchange station with a longer than normal pedestrian connection. Less than Spadina, more than others. And that's just a vague placement of the connection. Optimally it could be more direct.
And yea gain storage, gain another line, improve Union capacity, provide direct service to the waterfront...