Possibly ... in theory one could build a short tunnel somewhere along the east sidewalk of Main Street and under the road, that connects from the Mezzanine of Main Station to a portal on the pathway to the GO station ... or even to the tunnel under the tracks if you slope it enough. Not sure it would ever happen.
They have recently added a pedestrian pathway along the east side of the tracks to Esther Shiner Blvd, and then you just walk along Esther Shiner and Old Leslie; it's a 5-minute walk, and you can see it clearly now on Google Maps. However, the original plan had been to move the platforms a bit further north, though I think that plan has now died with the construction of Esther Shiner Blvd. You could of, however, had moved the platforms a bit further north, so the south end was near the parking lots under the 401, and the north end was near the GO station, and easily connected into the GO. There's a lot of room for development though south of the TTC station on the east side of the tracks - a better connection one day is quite feasible. And that new complex on the west side of the tracks, on the north side of Esther Shiner will be connected into Leslie Station when it's completed.
Though the current Metrolinx plan doesn't show Lesle/Oriole as a Mobility Hub ... nor Dowsnview Park (aka Sheppard West) oddly enough.