Yes I think a great route would be from Portsmouth Harbour towards the West Country. Obviously I know it would be to large for one route but maybe in sections starting from Portsmouth Harbour
Portsmouth Harbour - Southampton : 25.75 miles Portsmouth Harbour - Salisbury : 50.5 miles Portsmouth Harbour - Westbury : 75 miles Portsmouth Harbour - Bath Spa : 91.75 miles Portsmouth Harbour - Bristol : 103.25 miles
Cant find any trains which go from Portsmouth to Exeter or beyond. You have to change at Salisbury or Westbury
The distance would be to great for them to do it in one go. So Salisbury or Westbury could be the end of one of the routes and the start of another.
A lot of DTG TSx routes are 100 miles or more. Admittedly the newer ones seem to be shorter, but they've just announced EUS - BHM at 113 miles, so portsmouth to Bristol is doable, especially as it's much less sparsely populated than the WCML Split into two would still work, and probably at Salisbury as it's a four way split there whereas Westbury is only a link with the GWML