OK the full route from Marylebone is out of the question but have it from (for example) Rugby Central to Nottingham Victoria via Leicester Central would he decent. Stock would/could be 9fs on freight. Older Diesels and DMUs as well.
Any bit of the GC would be good though my personal favourite is the section between Aylesbury and Rugby so you would get Brackley Viaduct, the yards and loco shed at Woodford Halse, Catesby Tunnel and the following viaducts at Catesby and Staverton Road. (Not forgetting Helmdon Viaduct at, erm Helmdon).