Welsh Govt has just recently brought a large plot of industrial land near Taffs Well station, which it has been suggested could be a new 'Metro' depot.
Interesting. Unfortunately, going from Google Earth there looks to be a few tight spots between Radyr and Taff's Well where 4-track wouldn't fit. That would mean, if Taff's Well was a tram depot, everything north of there would also have to be tram. Tram-trains might work though.
IF you could 4-track Radyr to Taff's Well, you could convert Cathays-Radyr to tram and send Aberdare/Merthyr/Treherbert services via Danescourt like so (working on the assumption of 15min intervals on most of the network, and ignoring, for now, my concern about pathing on the GWML between Cardiff and Bridgend):
Treherbert - Danescourt - Rhymney (4tph)
Merthyr - Danescourt - Blackwood (2tph)
Merthyr - Danescourt - Trelewis (2tph)
Aberdare - Danescourt - Blackwood (2tph)
Barry Island - Abertillery (2tph)
Maesteg - Ebbw Vale (2tph)
But that still leaves some Aberdare (2tph), Maesteg (1tph), Swansea (1tph)
Bridgend (2tph) and Barry Island (1tph) services with no electrified destination to go to east of CDF. You have Chepstow (1tph), Cheltenham (1tph) and Hereford (1tph) as bi-mode destinations, but that still leaves 4tph to terminate at a rather busy Cardiff Central.