So I made a design which hits all the areas Gweed mentioned. It utilizes the
Don Branch of the <strike>Bala Sub</strike>
Belleville Sub and a redesigned Half-Mile Bridge from just north of Broadview station to about Laird Dr. Although the route looks a bit wonky, it is more direct than if it were to somehow use the Richmond Hill line. East of Thorncliffe Park station the line descends underground below a hydro corridor and uses another bridge to cross the West Don River, before reentering the valley and intersecting with Don Mills station.
Three large bridges, a significant portion running in the open air, shallower stations, Leslieville and south East York not served... Clearly there are pros and cons. But are there cost savings? Is it realistic? Would NIMBYs be opposed? Who knows.
Edit: Perhaps what I'm calling the "Bala Sub" is actually referred to as the Don Branch or Leaside Spur or Havelock Line. And I guess Metrolinx has looked at this particular routing as its mentioned here:
http://www.metrolinx.com/en/projectsandprograms/reliefstudy/YRNS_Long_List_for_Consultation_EN.pdf
Double Edit: Yes, fucked it up. It's the Don Branch of the Belleville Sub. I believe Metrolinx does own it.