With regards to South Wales/VOG, I wouldn't say GWR need to go via the VOG. This would be better for TfW to introduce;
GWR - Extend South West - Cardiff services to Swansea (158/5 car IET) that also calls at Pyle to give people from South Wales direct access to the Southwest.
TFW - Cheltenham services go via the VOG to Swansea or even extend to Pembroke Dock in summer via the district line to serve the proposed new station North of Swansea. West Wales / Pembrokeshire is woefully underserved considering it's such a big tourist destination. This service only calling at Barry, Rhoose and Llantwit Major to give people from West Wales access to Cardiff Airport. There's very little demand from Southwest England for Cardiff Airport. Barry Island is very popular on hot days in the summer which is often in high demand from stations on the Cheltenham line.
TFW - Maesteg services run through to Ebbw Vale instead of Cheltenham which will still serve Pencoed, Pontylcun and Llanharan.