I think a SWR route would be great especially something like the Portsmouth Direct Line (like the one in Train Simulator). The line would run between London Waterloo and Portsmouth Harbour with the rolling stock being the class 450 desiro and class 444 desiro. This route could connect with the London Commuter route at Clapham Junction allowing for a more packed experience for both of the routes. I think this would be a great route and we need more modern era British routes in TSW2.
This would probably require the route to be set a few years back, but it'd be really cool to include the Class 442 Wessex Electrics as well, since SWR previously operated them on the Direct Line from 2019 to 2020 until the pandemic led them to all be withdrawn.
We do need more Modern English routes but just not in the southeast. The midlands and East Anglia need to get some love!