@Streety McCarface how familiar are you with Coxwell between Upper Gerrard E and Danforth? Despite what Google Maps on iOS says, it ain’t “mostly flat” and tunnelling it to run at a shallow incline and passing beneath Line 2 to a terminus near/past East York Civic Centre would almost certainly require continuance of a surface route to serve the two schools and the residences in between.
One notion I have had is to eliminate the 502 in favour of 503 as the sole Kingston Road route, and replace it with a bus route along Dundas as far as Toronto Western. There are several growing nodes along that route (Ryerson-Jarvis, Regent Park, Dundas-Logan) and potential ones (the cop shop being vacated at Coxwell) and it would take some pressure off the 505. The 22 remnant Coxwell section could interline with 92 (via Lake Shore) or 31 (via Eastern)