Perhaps a service to Bath could compete with the X72 bus services? Buses are an outdated form of transport, and it would be easier to walk to the station for some people rather than into the town centre....I vaguely remeber hearing something about a problem with "doubling back"...but then, the trains to Cardiff "double back" when they reach Bristol Temple Meads.
Various options have been / could be looked at for that, but it's probably not the first stage.
Trains can be reversed at Chippenham (for sure) and Trowbridge (I think) but in both cases via the running line rather than a bay or extra platform, which would mean that the operator whould want to be very sure of robustness in order to avoid delays and potential penalties.
Looking forward, Network Rail have it in their plans to put a track back against the spare platform faces at both Chippenham and Westbury, so that coule eliminate the robustness issue at Chippenham and also help with Westbury reversals - platforms appear to be something that they're NOT short of at Bristol Temple Meads from what I can see.
Further forward, I hear "Bradford North Curve" being muttered!
But ... are First [Great Western] going to want to compete with First [Bus] - somehow I doubt it.