TfW running to Birmingham has a lot to be said for it.
However, the current schedule has the Cardiff to Birmingham arriving at about xx:45, but the Birmingham to Cardiff is xx:30. There would need to be some form of recast to make a reversal termination work.
In all cases a different path would need to be found that still allows sufficient turn around time in Cardiff (or extends beyond Cardiff).
One option to avoid New Street termination is to run via the Camp Hill in one direction. That severs University stops in one direction.
Another option is to terminate at Moor Street. Moor Street termination would be convenient for onward HS2 connections (e.g. to Manchester/North West), but is likely to do little for other connections. Also cuts off University stops.
Without consulting the timetable, is a wildcard option that the stock from the Shrewsbury* train continues around Camp Hill to go to Cardiff, whilst the inward working from Cardiff goes to Shrewsbury*? That would cut off the Birmingham Airport direct service from places on the TfW route, but maybe WMT could extend their trains if that airport connection is so important. I would have thought Manchester Airport serves a lot of people in Shropshire (and does not have a direct train).
Worth saying this would need more 197s so is probably a complete non-starter no matter how much I would like to see more 197s.
CrossCountry having 170s interworking Nottingham and Leicester* services at New Street is straightforward enough and may allow for diagrams that do not have such a tight turnaround from the inward Leicester forming the outward Stansted (for example). The saved 170s could either strengthen services, or create an extension from Leicester to Nottingham and allow interworking of diagrams at Nottingham as well.