I think it would be a cool feature to allow players to either, Create their own timetable with whatever trains they would like to put on it. Or allow players to choose to put trains from other routes on that specific route instead of doing it in the scenario planner. I would like this feature so when the Class 378 with Southeastern HS comes out, I can put it on GWR.
I partially wrote a timetable for Brighton mainline for Train Simulator. It would be great to see this in TSW2.