Optimal route for the DRL North to take is to meet the Yonge Line somewhere north of Steeles. You maximize relief to the Yonge Subway by intercepting riders coming from the north of the Yonge Subway (not to mention providing York Region commuters with a much more rapid route downtown than the Yonge Line). I'm going to link a post in the Crosstown thread which covers these same points
here.
The question is more so, if taking over the Richmond Hill GO line is feasible. It being looked at as Option 3 in the present study lends some credibility to the corridor, though I would prefer an alignment like
this personally.