Personally, I would prefer they continue to build upon the modern era GWE route. For example, for locos they could add the 800, 387, or even the new TFL rail trains. In terms of extensions, DTG could add all the short branch lines to add some diversity to the services or even extend the route to Oxford using the existing services. All that probably won't happen since GWE was literally the first route DLC for the TSW franchise and is showing it's age. The focus at DTG is likely on the future.