I think the plan is to come off the City Line just north of Fairwater station and then follow the old single track railway out towards Cregiau. As this route was destroyed near J33, they might use St.Y Nyll lane underpass to get north of the M4. Then through the new Llanilltern development area to reach the former dual track line that once crossed the Ely Valley at Drope (near Culverhouse Cross). The route would then reach Cregiau and get to the north side of the A4119 by using the present underpass still in place on the old railway.
What they should now be doing is determining where the stations will go and how they best fit with the new developments as well as serving older settlements such as St.Fagans and Rhdylava with Park & Ride at nearest locations (eg.Croft-Y-Gennau Road).
You will note that mention has been made of a reconfiguration being needed at Cardiff west junction - near the Canton depot in order to increase the number of services on the City Line (west) from the present half hourly. This is absolutely vital and is of course, on the section owned by Network Rail. It surely would be beneficial to have this route electrified between Central & Ninian Park as that would enable GWR to run Electrostars as soccer specials direct to Ninian Park station - thus freeing IET’s for regular passengers.