North Wales Coastal would be easy and popular. All they've got to do is debrand the Arriva rolling stock.
Midland mainline st pancras to Bedford would be nice. They've got the 43, would tie in nicely with additional 700 DLC for BML
Already available from DTG. See https://store.steampowered.com/app/..._Midland_Main_Line_LondonBedford_Route_AddOn/