I do not know Montreal that well. I see the logic in continuing the Blue line along Rue Jean Talon, instead of finding a way to switch to a different alignment.
Perhaps Montreal needs a Downtown Relief Line (LDCV) (Ligne de Décharge du Centre-Ville) that goes down Papineau Ave and across on Rue Sherbrooke.
If I'm not mistaken, right now the biggest project downtown Montreal would be an extension of the Yellow Line to McGill Station to relieve Berri-UQAM (the main transfer point). Especially if the South Shore extension happens, dumping more passengers off almost, but not quite in downtown is going to create an even bigger problem than currently exists there.
It would be a big project, but nowhere near the scale of the DRL. Remember, Montreal already has 3 independent lines feeding into the downtown area (well, 2.5, because the Yellow line comes close but not quite). Toronto has 1.