one category in TS2018 that has never been attempted is the world of miniature railways these little railways could be a quick and cheap project due to the length of the lines they run on such a the Conway Valley Railway Museum and Rhyl miniature railway these little railways have often unique locos and rolling stock with short but detailed routes if available in TS2018 this could be a cheap alterative to long and time consuming builds when you can build it shorter in less time with the same experience from a regular route
I think that the Romney, Hythe and Dymchurch Railway would be a good suggestion. 13 and a half miles plus 16 engines (13 engines being the main fleet and 3 being shunters) would give a great variety for users to drive.