When it comes to a western extension of the OL I have been thinking of perhaps something a bit more unorthodox. Assuming we don't just follow the DRL plan and go north up Roncessvalles to Dundas West here's 3 possible western extensions.
OPTION 1: Extension to Sherway Gardens via Queensway
For this extension the route follows the Lakeshore West line to Queensway and then runs along Queensway to Sherway Gardens. Stations would be at: Roncessvalles, Windemere, Royal York, Kipling, Sherway Gardens. Perhaps Line 2 also receives a 2 stop extension west to meet the OL at Sherway creating a new transit hub. Of course this also leaves open the possibility of a further western extension into Mississauga although due to the difference in technology only 1 line could be extended further west while the other terminates at Sherway. The main issues I can see with this extension are:
1) to much service for Queensway which may be better served by a streetcar
2) to close to Line 2. It can be argued that this extension may be to close to Line 2 and could cannabilize some of its ridership. This may be a good thing though since it could relieve growing pressure on the western half of Line 2 with riders commuting from the west end and Mississauga chosing to use the OL to get downtown instead of Line 2.
OPTION 2: Extension north to Pearson via Kipling Hydro Corridor:
For this extension instead of heading to Sherway Gardens, the OL turns north at Kipling and heads north via the Obico sub and Kipling Hyrdro corridor to Pearson Airport replicating part of the unbuilt Etobicoke RT. Stations on this extension would be: Kipling (Line 2), Burnhamthorpe, Eglinton (Line 5), Carlingview, Pearson Intl Airport. This extension would help alievate the lack of the major north-south line in the cities west end. The main issues with this extension are:
1) Construction Complexitity. The use of a hyrdro corridor pretty much precludes the use of an elevated structure thus the line would need to be built either under the corridor or in a trench. This is further compunded by the mess around Eglinton since there is both a large substation, gas lines, and Line 5 in the area meaning the line would have to be deep bored through here.
2) Duplication of UP Service. It can be argued that this extension would duplicata the primary role of the UP. While the UP would still be a faster ride into downtown, I have no doubt some would use the OL instead if only for its lower TTC fare.
3) Duplication of Line 5 Service. It can be argued that this extension would make the Line 5 extension to Pearson usless since you could just transfer to the OL at Martin Grove/Eglinton. Perhaps this make a case for Line 5 to be extended west into Mississauga via the BRT corridor instead of to Pearson.
OPTION 3: Turn the OL into a Ring Line via the Humber-Pearson & Finch Hydro Corridors.
For this extenson instead of heading to Pearson the route continues north via the Humber-Pearson Hydro Corridor and then east via to Finch Hydro Corridor to Don Mills Avenue, were it then turns south and reconnects to itself forming a ring line. This would fully replicate the unbuilt Etobicoke RT and give the cities west end its missing north-south line. Stations of this extension would be: Martin Grove/Dixon, Kipling, Islington, Finch/Weston (Line 6), Jane, Finch West (Line 1, 6), Dufferin, Bathurst, Finch (Line 1), Bayview, Leslie, Don Mills/Finch, Don Mills (Line 4). The main issues with the extension could be:
1) Construction Complexities. Just like with Option 2 all of the same issues with construction in the hydro corridors exist here.
2) Duplication of Line 6 Service. It can be argued that this extension would duplicate some of the service provided by Line 6. As well you can make the argument that you can get the same or similar results by extending Line 6 east to Yonge Street and beyond.
Of these 3 options I think Option 1 is the safest bet since it has the fewest complexities and may be more in line with the Provinces goals for a wider regional transit network (i.e. extension to Mississauga).