My preference would be to install a streetcar service along Parliament from Castle Frank to King, and then have it cut westward along King, and then up Spadina to Spadina Station. Basically, you'd be forming a slightly wider and shallower U compared to the YUS U. With a hopeful King Transit Mall, it would mean 2 of the 3 legs of the route would be some form of ROW.
The current issue with the Parliament bus is that it doesn't really go anywhere where people want to go. Most travel is towards the core, which it doesn't really serve. Having it curve westward into downtown without a transfer would also take a fair bit of traffic off of the Carlton, Dundas, and Queen cars, as well as the Yonge Subway.