With a magic money tree, I'd want a bunch of 5-car bi-modes (with DC, AC, and diesel*) for the Portsmouth-Cardiff run, with a similar unit without the AC capability for the Waterloo-Salisbury-Exeter line. Think along the lines of a bi-mode 444. Would make sense to electrify out to Temple Meads, and eventually the Swindon-Bath-Bristol route, so eventually they'd only be on diesel between Bath and Southampton (and IIRC a small section near Fratton?). I'd also want to see if it were possible to alleviate some of the overcrowding on the route by increasing frequency to half hourly, although I'm not sure that's possible with pathing.
For the more regional lines around Bristol, some smaller bi-modes along the lines of the 755s might be a good option. But it all depends on electrification spreading out.
In a more realistic world, I think a proper refurbishment of the Turbos is probably the best option in the near term. Longer term, I could see an order of some bi-mode units to replace assorted Sprinter/Turbo units knocking about. If the Chiltern Main Line were to be electrified, cascading some 168s would be a possibility although they're not that much newer than the 165/166s. They could be handy for replacing Sprinters though!
*bi-mode as in diesel or electric, keeping going from GWR's 769s being 'bi-mode' although they can run on diesel, AC, or DC.