FGW▸ has depots at Fratton and Bristol ... and SWT▸ at Salisbury. There would seem to be a degree of sense in the longer term in having services which originate in Wiltshire served from Salisbury.
FGW also have a sizeable traincrew depot at Westbury, also in Wiltshire. There is also space at Westbury to build a small depot capable of fuelling / FPX should the need arise. No requirement for Salisbury (which will probably be fully stretched when the Waterloo - Exeter goes hourly in December) involvement in the longer term on current FGW operated routes.