Driving HST you can only go 1 stop to reading extending this route would be good with some sections of these routes. Paddington to Cardiff/Carmarthen, Bicester village via Oxford and or Penzance via Bristol
I would like a GWE extension, however, Cardiff, is unlikely, not for a while yet anyway. Penzance, is far too long. There is a suggestion (I will link below) about some branch lines, which is interesting and do-able.
This has come up and been suggested several times. The impression I got from previous discussions is that DTG are at best lukewarm to the idea. We don't know whether routes created in their UE4 editor can be simply extended as you can do with a route in MSTS, Trainz or TS20xx or whether it would have to start from scratch. There is then the commercial side of the thing as to how you sell it - whether as an extension to Paddington to Reading if you already own it or as a complete route to Oxford, Swindon or wherever if you don't. Personally it is something I'd like to see, as it stands the existing GWE was pretty much a proof of concept for the game's early stages and far too short compared to more recent projects.