6 coaches - four standard, 1 buffet, 1 first. Sorted, maybe some longer sets on the busiest services. I'm sure the Waterloo - Exeter is well overdue an upgrade, there was talk in the late 90s about converting it to an IC railway to compete with the GWML. I remember getting a SWT newsletter detailing all the plans, we could've had Class 175/180 style trains running to Exeter by now. Most of the stops further west would've been ommited by the London trains and provided by a 'stopping service' (probably a 150 back then, now if done most likely a 159). Now seems like the best time to consider it again regardless of who gets the GW franchise. The FGW HSTs would be refurbished with power doors, ending up with an interior similar to the EMT 158s. The 159s can only go up to 90mph and wouldn't be able to take advantage of any speed improvements, so they'd have to go elsewhere. A Waterloo- Weymouth HST via Yeovil service with speed improvements may be even quicker than the mainline. If Chiltern and Virgin can compete with similar timings using IC trains on London - Birmingham, GW and SW can too.
A couple of points:
The stopping service would have to not run into Waterloo or be after Crossrail 2 (regional) if that ever gets built. That would likely mean starting/ending at Salisbury or Basingstoke, unless there was a link to a branch line from the country end that it could run along (which, other than going to Reading, I can not think of any before Woking and that's where a whole load of additional trains join the Mainline). Either way I would suggest that the passenger numbers on the stopping service would be low.
The other way of doing it would be electrify to Salisbury, extend the (EMU) stoppers between London and Basingstoke through to Salisbury, retain the existing Exeter Service and replace the current Salisbury service with IC125's and run them fast (London, Basingstoke, Andover, Salisbury and Yeovil) to Exeter. I would suggest that there was enough demand to not call at any other stations between Basingstoke & London as that would improve journey times and lighten some of the loadings on the other services out of Basingstoke. However that would require a whole load of new cabling and trains running "under the wires" for a lot more than at present.
To Run Waterloo to Weymouth Via Yeovil would require a new junction at Yeovil missing out the current stations or for trains to reverse at Yeovil, which would remove any benefit from the new service.