Near the Junction ... somewhere near Dundas/St. Clair/Keele I'd think. Which would raise the interesting possibility of terminating the 512 streetcar there.
In a fantasy world, they'd extend 506 up Parkside/Keele and terminate that there as well, and stop the Keele and Weston buses at St. Clair.
Normally when people talk about where they relief line should go next, the thought is always north or up the Weston tracks to Mount Dennis. But what about turning west there on either St. Clair, Dundas, or in between? It's got pretty good density to Jane. Mount Dennis will already be well-served with GO, UP, and the Eglinton line. Perhaps one day it can go west into Mississauga. Meanwhile turn Line 2 north to the Airport via Renforth station. I guess this is in fantasy map territory ...