I'd go for the East Coast Main Line with modern units or semi modern units. I'd like to see the mid-section though through Doncaster and up to Leeds.
ECM, WCM and Frankfurt High Speed aka Köln-Frankfurt. After that it would be Portsmouth Direct Line, Bournemouth-Southampton, Rhône Alpes
All routes on the list are worth building and will be interesting for the community. But London-Brighton is the most worthy. Classic Tc deserves the highest respect.