Beyond what we have, what is under construction and the identified priorities that nfitz mentioned I think the only remaining corridors in the city that warrant any rapid transit of any form whatsoever probably narrow down to:
Finch east
Sheppard from Yonge to Downsview
Eglinton East-Kennedy-UTS-Malvern (maybe)
Queensway (not much existing ridership, but a lot of development potential)
Maybe rebuilding the section of the 501 from Sunnyside to Long Branch to Transit City standards/gauge.
Beyond that, everything else strikes me as being somewhat superfluous and a "nice to have", but not really necessary.