Epsom Downs and Tattenham Corner can accommodate 10-car trains (but the platforms at intermediate stations on the Tattenham branch are short)Tattenham Corner, Caterham and Epsom Downs are all 8-carriage branches. Could there ever be demand for 12-carriage trains on any of these lines? If demand on the core increases, TL will have to become all 12-car, so any thoughts about where its branches should go should focus on where an all 12-car TL would make sense, rather than trying to make its extent match the London boundary so that TfL can take it over.
However, none of these routes need 12-car trains to the Core. Moreover, there doesn't appear to be capacity on the slow lines through East Croydon to run separate services to Caterham and Tattenham Corner (which is why Thameslink didn't take over these routes) although the removal of the Victoria service does help in that regard. There will never be 12-car demand on these three routes of course (but it is perhaps more about capacity between East / West Croydon and the core than the branches).