So here would be my proposed alignment for a Parliament DRL:
I figured since it would have to dig deep to go under the Bloor-Danforth line at Castle Frank, it could just continue tunneled deep under the Don River and emerge on the alignment of the old and empty Belt Line rail. It would be bridged over the Bayview Extension and elevated/surface to Millwood where it becomes tunnel again under Thorncliffe and Flemingdon Park and onto Don Mills. In downtown it would follow a Parliament alignment before turning west on King (or whatever) giving us our downtown tunnel.
Yes, this is not as glamorous as the traditional DRL alignment through East York, but the traditional DRL alignment is prohibitively expensive, crosses the Don River twice, has no answer for how it would align from valley to cliffside at Thorncliffe, and earliest estimates have it ready for 2035 - too far off for relief needed yesterday.
This alignment is cheaper, easier to construct and has less engineering challenges and is therefore a more realistic DRL option. Yes, it misses out on East York and the density near Pape, but I'd wager that the east end of the downtown core, Cabbagetown, Regent Park and Corktown, are even bigger fish with greater employment and development potential while simultaneously relieving our downtown streetcars. HOWEVER, with a smart SmartTrack alignment&configuration, we can still yet provide rapid transit to East York. (I am interesting in ideas of incorporating the downtown tunnel with SmartTrack/GORER)
One problem with the Parliament alignment however is I am not certain how much relief it provides for the Yonge subway as unless your destination is south of Queen station, it is probably not much quicker for westbound travelers to transfer on Parliament. Perhaps it being less congested is incentive enough? Perhaps it doesn't even matter? I always felt the benefits of the 'relief' aspect of a new downtown line to be oversold compared to all the other benefits anyway as not much could actually relieve the Yonge line besides a new express subway on Yonge or a GO RER alignment that absorbs all the York Region commuters from the Yonge line.