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.
Also perhaps a commuter line like Waterloo to Weybridge or Waterloo to Windsor and Eton Riverside, that would be amazing!
Yes we need more modern English routes but we really don't need more routes in the southeast. The Midlands and East Anglia need some love.