Plymouth To Bristol with a IC Swallow Valenta HST, RR 150/2 & 158 and a reskin - ICS Class 47 / Mk2 stock would be fab. Better still Penzance - Paddington.
How about Penzance-Taunton-Reading. At Reading you're going to get this pop up do you like to continue to London Paddington yes or no yes option is chosen you actually boot up Great Western Main Line Paddington Station to reading then continue as normal.
Except for detail level Being able to wander around (so more requirement to make tunnels usable etc Development costs being generally higher So besides those points, no excuses no...
Both these suggestions, while they would be nice to have, are not going to happen anytime soon. The London to Penzance route in Train Simulator was not even made by Dovetail. While TSW (probably) can handle a route of this length, the limit on consoles is the problem. As well as that, the size would be HUGE. On PC, GWE London - Reading is 11 Gigabytes