Bit late to the party on this thread, but I'm a strong advocate for diverting services away from Euston to Camdren Road, and sorting Willesden Junction out once and for all to provide capacity for Barking services to Richmond. Access from Kensal Green, Queens Park, & South Hampstead to Euston would be by changing at either Willesden Junction or Chalk Farm/Primrose Hill (linked by a new passageway) to WCML slow lines or the Northern Line, respectively. Or just hopping on a bus - it's hardly very far. As suggested upthread, you make paths for Goblin services through Hampstead Heath by routing some NLL services via Primrose Hill. Ideally, I'd like to see most if not all of the freight shifted from the NLL to the Goblin by altering the connections to the GEML near Manor Park, though there's (or at least, there used to be when I last looked) plenty of room even with a reinstated island platform to add a pair of long freight loops at Primrose Hill.
As a bonus, here's a bit of crayoning I did many years ago for Willesden (lighter sections are what you'd build immediately, and darker sections what you'd leave space for adding later):
As a bonus, here's a bit of crayoning I did many years ago for Willesden (lighter sections are what you'd build immediately, and darker sections what you'd leave space for adding later):
