In a similar vein to the Oakville route, it would be neat to see some stuff surrounding Toronto's CN MacMillan yard. From there you can head east down the York Sub to Pickering Junction where it meets the Kingston Sub with GO transit and VIA traffic. A bit east of Mac Yard is the Newmarket sub taking GO transit traffic north towards Newmarket and south to Union Station. Further east down the York is the Bala sub which takes freight North to Capreol, and GO transit traffic north to Richmond Hill and south to Union Station. West of Mac Yard to the Halton Sub seeing GO transit traffic going to the Weston Sub towards Union Station. Many of these also see CN local jobs running out of Mac Yard to various industries. Obviously there is a lot there, and may not be feasible for 1 route, but some sections of this would definitely be cool to see. For example, the Lakeshore GO Transit route from Oshawa (Which also has a CN yard for local traffic from Mac) through to the currently existing Oakville route to Aldershot via Union Station. My interest in this area is due to my employment at CN in Belleville, from which Toronto is our westbound destination.
I'm making the kingston and belleville sub from Oshawa to Cobourg. Sadly it is going to require some more obscure dlc for the textures but it should be pretty good looking hopefully. It's my first attempt at making a route based on a real line.