YES will be seriously considered once the DRL is built and is deemed insufficient at diverting enough users away from the YB interchange.
I predict that someday an entirely new line, roughly paralleling the current L1 Yonge section, but buried much deeper--and boasting double sided platforms to allow detraining on both sides, and 660' long stations for up to 10 car trains.
I see this eventually being built from RHill to The Portlands, with interchanges to line 1 at Finch, Shepherd, Eglinton, Bloor, Queen and Union, then continuing as a solo line through the East Bayside, West DL and terminating at the SE .end of the Portlands. (or in my fantasies, the E end of The Beach.) YES would run deep under Bay street in the CBD with multiple Path connections from the Bloor, Queen and Union interchange stations.
It's a pipedream that would likely cost +20B but, my god, what a difference it would make to the well being and liveability of this city.
It could be marketed as Canada's great urban RT line for the 200th anniversary.
With a surfeit of fast, new convenient high capacity RT spreading through the core it will mean the end of private vehicles as a viable source or appropriate means of transit in a super dense, multi connected rapid transit dominated core. Only buses, taxis, trucks and emergency vehicles would be granted access to the new CBD/DT.