I think adding some more routes to the London Underground would be a popular choice maybe even some football scenario routes for football fans with AI British Transport Police and AI in football shirts ect that would be awesome and maybe even maintenance on the lines with delays and stuff like that so feels more realistic..
Which sim do you want? This is the TS2020 forum. If you want TSW2 it is here. https://forums.dovetailgames.com/forums/trainsimworld/