The cornish Main line from Penzance through to Plymouth useing the Class 43 Hst, some form of DMU and maybe even the Class 66 for some Clay workings.
https://forums.dovetailgames.com/threads/cornwall-pnz-to.22352/ this is where the biggest Cornish Mainline group. We are pretty near all in agreement that it should be added. It would be an amazing addition.