I have been wishing for this for weeks. Since this route has a 'Via Norbury' branch, I would like Dovetail Games to do services via Crystal Palace and the extension to Sutton. To add to that, I would like a Class 455/8 Locomotive in Southern livery. Hopefully that will be fine.
It might be better as a separate route with the Class 455 like South London Network on Train Simulator. DTG have said they don't plan on doing route extensions, as separate routes don't require base DLCs for them to work. The East London Line to Sutton including London Bridge, Crystal Palace and New Cross might be a good fit.