Thos suggestion is for extra stock on Tees Valley Line and would comprise of... Class 56 in various Railfreight liveries (possible layer on GWE). Class 60 in Railfreight sector liveries (possible layer on GWE). Class 142 Pacer in provincial livery. Class 143 Pacer in provincial livery. These classes would provide many more service opportunities and improved playability.
No reason why it shouldn't get another. And all the traction types stated by Elarthur have huge potential for layers on future routes, especially the freight locos.
Also the Class 142 & 143 pacers because these are designed to couple with Sprinters and Turbostars except for Class 171. Have TVL Pacers with West Cornwall Local you will find a Class 142 & 143 Pacer behind the Class 150/2 DMU. Reason for this is that Sprinters Turbostars except for Class 171 and Pacers use BSI Couplers.