I may be smoking something worse than what the MTO already does, but I would number it 413. My 413 would begin at the 404, continue west to the 400 where it would turn south running concurrently with 400 until it turns west again as 413 to the 401/407 junction. In my fantasy mind, I would end 407 at that junction with the 401, renumber the 407 and 403 south to the QEW as 413, and renumber the portion of the 407 from the junction of 403 to the QEW in Burlington as 403 as originally intended back when corridors were first identified back in the middle of the 20th Century. All this would make the 413 a freeway from Queensville in eastern York region, arching around down to Oakville