I suggest Tavistock is one of the less improbable re-openings. There is an under-used but intact railway line for most of the way and a largely clear trackbed for the remainder, although it is unfortunate that the formation beyond the edge of Tavistock is blocked. Although there is a direct road to Plymouth (probably the reason Tavistock was closed while Gunnislake remained open) it gets very congested. Saying that Tavistock is within a few miles by road of a railhead is irrelevant: Bere Alston has terrible road access and both road and rail legs of an end-to-end journey via a park-and-ride at Gunnislake would be very slow and unattractive even compared to the current road journey into Plymouth. Okehampton may be different. It also has a surviving railway so it should be relatively easy to restore service to Exeter, but it hasn't happened other than various Sunday services and specials. If it was able to sustain reasonable passenger numbers on an hourly all-day service then the case for re-opening beyond might look stronger. However extending to Bere Alston would cost a lot more as there is no railway now, and it's not obvious that the benefits would be any greater than a regular service to Exeter. Like all re-openings the benefits of a diversionary route are small because whatever benefit there is has to be divided by 365 and multiplied by the few days a year the diversion would be in use. I can't see any reason for Plymouth-Exeter to be any quicker via Okehampton and if there was significant demand for through journeys to the Waterloo route there would be some already (they have operated via Newton Abbot in the fairly recent past). Suggesting that the restoration of Okehampton to Bere Alston makes journeys between Plymouth and Basingstoke more likely is basing an argument simply on historic service patterns with no consideration of what might have changed in the past half-century.