To be honest, I don't see much demand along Sheppard for the stretch to the Airport, and expecially none past Jane.
What I'd do is interline Sheppard with the Spadina extension to York University station. That way, YUS trains could short turn at Downsview like they do now, and Sheppard trains would run the full route. This allows a seamless route to York University and the Finch LRT, which I think can be quite valuable in the future.
For an Airport route, I'd seriously like to revive the GO ALRT route. The Finch hydro corridor would mean a rather inexpensive route, which'll be a pretty big draw for such a long route. I don't think that it would overlap service with Sheppard, but Sheppard should definitely be built first. After Sheppard is totally finished, Metrolinx could assess the need for another, more regional route.