If you look at the map, the route is largely through rural areas - between Fairwater and Creigiau is rural, for instance. And then the route goes north west from Creigau through largely rural areas towards Cross Inn. You couldn't have a completely new town, but you could expand existing settlements and have a series of new "villages" along such a route.
I prefer a development that is contiguous with Cardiff - because I think this will be more feasible to develop in stages. For instance, the first houses could be built before a primary school was complete. A development further away would need additional infrastructure sooner (which is difficult given today's spending environment).
Nice idea, trouble is someone has forgotten the price of land that has planning permission and that a garden city by its very nature will have a lot of open space. I also think there are targets for housing density still.
So the development will have loads of flats and no matter what planners might say people still want a house.
Potentially the houses are going to be too expensive, unless the council owns a lot of land and effectively gives it away, or house builders start building townhouses to gain the maximum utility from the land they can build on.
The devil will be in the detail, will they plan for the future around the needs of young families (or those wanting to start families) or those who are 50+ and whose children are flying the nest (and hence make an easy profit)?