There'd have to be a spur into CWL as well, because the existing Rhoose Halt with its shuttle bus might put off some users.
Currently there are two routes between Bristol and Cardiff (apart from the GWR HSTs that call at Bristol Parkway): from Taunton/Weston-super-Mare and from Portsmouth/Southampton. They both call at Bristol Temple Meads and stop at the main stations between Temple Meads and Cardiff Central.
If there was sufficient rolling stock and pathways - there almost certainly isn't available rolling stock at the moment - some of these trains (probably those from Taunton/WsM) could be extended to Rhoose now but there is unlikely to be sufficient demand from the West Country at present. When the Qatar service beds down it could be looked at again but my feeling is that CWL needs a proper rail connection to the airport itself to make rail a realistic option.
Even 2-3 mppa might not be enough throughput to justify the cost of a new spur and station. It's the age-old problem of the chicken and the egg. In this case if the airport doesn't grow sufficiently to make a rail connection viable it might not get one; if it does get one a spur and station might then help to take the airport to a situation that justifies such a link.
One of the most successful small airport rail connection is at Southampton where the line from Waterloo passes the airport. Built around 50 years ago there was no rail connectivity required to be built, only the station, and although SOU has just reached 2 million passengers in a 12-month period for the first time the station is apparently well used.
Prestwick with a rail connection from Glasgow Central has gone the other way. Not so many years ago it was handling 2.5 mppa. It's currently around 700,000 pa.