I thought you aspired to a Swindon-Salisbury shuttle!
We aspire to an appropriate TransWilts service - decent service Salisbury - Warminster - Westbury - Trowbridge - Melksham - Chippenham - Swindon. If the practical steps to that mean a good, same-platform connection at times (and no time-consuming dogleg), that's not a problem. And if there's a bonus of serving other logical places that happen to be just outside Wiltshire, again that's good news.
There is so much sense in maintaining and servicing a mainline TransWilts unit / fleet at Salisbury - except that a 153 or two would be "oddballs" at the
SWT▸ depot there and I'm not sure how that one could work out; I note that Stagecoach are pulling the plug early on the oddball units to Lymington. If we were speculating about recovering a 158 from somewhere, that might be sensible. So my "pie in the sky" proposal was to use folks who are used to looking after 'specials' - isn't there a mainline loco or two already based at Yeovil?
06:50 Yeovil Junction -> 08:33 Swindon
08:45 Swindon -> 10:01 Salisbury
10:11 Salisbury -> 11:33 Swindon
11:45 Swindon -> 13:01 Salisbury
13:11 Salisbury -> 14:33 Swindon
14:45 Swindon -> 16:01 Salisbury
16:11 Salisbury -> 17:33 Swindon
17:45 Swindon -> 19:30 Yeovil Junction
Funnily enough
the Westbury - Salisbury legs are at the "opposite" half hour to the Portsmouth - Cardiff, helping towards the aspiration of a half hourly service on all sections of that line. Some - such as the 13:11 ex Salisbury - take over from an existing service (ex Soton, I believe) potentially releasing that unit to bolster the TransWilts further towards the appropriate level.
I won't go to far down this analysis, as it's based on a difficult path; in reallity you would need several units on an independent fleet in order to always have one in service - I recall FGW▸ had a number of Adelantes for just a single diagram as they were phasing them out? It's also not been checked for single line occupancy, nor for a non-blocking approach from vested interests - better for all to 'work with' those who are already around than to do more than casually speculate about what could be done.