Forgive me if I repeat what others have said and for not sitting here reading 17 pages! haha
I've given it some thought:
An updated 00 MK3 and MK4 set of carriages
Perhaps an N and 00 Class 90
Something I have wanted to see for years is an updated LNER class B12
And if you want something completely different and off the wall how about breaking into the Irish theme? A class 80 would be a great choice, long lived, widely travelled and many liveries