With the extra capabilities of modern hardware, and with the upcoming Penzance - Truro and St Ives expansion, I'd like to suggest the South Devon Main Line route between Exeter St David's and Plymouth, potentially also incorporating the branch line from Newton Abbot to Paignton. The total mileage for the mainline and branch is just north of 60 miles, so not too massive but I do appreciate it is at the top end of capabilities. The route itself is stunning. You have the cities of Plymouth and Exeter adding some complexity and interest, but then you have the sea wall at Teignmouth and Dawlish, some really interesting gradients such as Dainton Bank, Hemerdon Bank nd Rattery Bank and the stunning scenery of Dartmoor as you head away from Totnes. There is scope for branch operations, with several stations on the route where GWR serve a fairly frequent stopping service using HST 4 coach formations and the new Hitachi trains. Historically Class 150s and other DMUs were frequent on these lines along with 8/9 coach HST formations. Also railtours could be a lot of fun - the Class 52 thrashing up Dainton is music to anyone's ears. If/when steam locos get into the game that would add a whole new dimension given the gradients. It's one of the best bits of railway in the UK in my opinion, and it absolutely needs to be in TSW. I'd love to be able to do services in my neck of the woods in game.
Hardware has little bearing on route length. It's the time it takes the devs to make the routes, trains and stations that sets what DTG will develop because they set themselves quite tight deadlines I'm sure the Riviera line will appear in game at some point because it's a classic steam route (and steam is one of their next big releases)
DTG have said they're not looking at route extensions for various reasons. We may well see Bristol, but not as an actual extension
The South Devon main line could be great, I actually made my own suggestion for it aswell, please do check it out
