Demand along Don Mills itself would come nowhere close to justifying rapid transit. Based purely on local demand, some kind of express or rapid bus seems most appropriate.
The question though is how would Don Mills interact with the surface routes it intersects. All of the Don Mills LRT studies were done assuming a fairly local style LRT which didn't attract much transfer ridership, not an alternative to the Yonge subway.
That said, the line doesn't even have to be on Don Mills per se. If ridership is mostly transfering, it lets the route be more flexible since we don't have to go out of our way to hit certain destinations.
There are dozens of permutations of these, but I'd see three major corridor options which could originate from Don Mills/Eglinton.
1.) Don Mills/DVP: The line could run above ground along the DVP, swerving a bit to hit things like the Sheppard terminus or Seneca College. The only other semi-significant trip generator along this corridor would be Lawrence-Don Mills.
2.) Agincourt rail corridor. Especially if the DRL approached Leaside from the West, it should be possible to build a low cost extension along the rail corridor to Lawrence/VicPark-> Ellesmere/Warden -> Agincourt-> STC or Malvern or so. Or it could turn up the Stouffvile corridor after Agincourt.
3.) Richmond Hill rail corridor. The line could be upgraded and straitened and stations added where major roads intersect.
All of those corridors would intersect the same basic feeder routes to some extent. It would take pretty detailed modelling to figure out how and at what cost. In terms of cost, they should all be fairly low and comparable. Stations would be relatively infrequent and routes would be nearly 100% above ground.
Ideally we'd actually see the DRL branch into two somewhere around here.
