With current stopping patterns, the journey time from Manchester to Swansea (or vice versa) is between 4 hrs 10 mins to 4 hrs 30 mins approximately. Extension to Carmarthen adds about 50 minutes (each way).
With a sensible turnround time at each end, it is unlikely that any set starting from Swansea will be able to make 2 round trips per day to Manchester, except perhaps the very first departure in the morning.
Extending through to Carmarthen during the day will essentially cost one extra diagram for a 2-hourly interval service, or 2 extra diagrams for an hourly frequency. Extensions to / from Carmarthen first thing in the morning / last thing at night should be possible. It is difficult to draw up any sensible version of the current timetable involving terminating at Swansea as the layover times there would be nearly an hour.
There might be some merit in an hourly Swansea - Manchester limited stop service, with a 2-hourly Carmarthen - Crewe sweeper, picking up the intermediate stops omitted by the expresses.
I guess we'll just have to wait and see what TfW decide to do. I'm pleased the Mk 4s are going to get some further use.