I was thinking about what a wasted resource the DC lines from Queens
Park to Euston are. A twin track railway to the edge of Central London
with only 3tph. But what to do with it?

You could build a curve from Wembley Central to Sudbury & Harrow Road.
The DC lines are on the west side here, so a flat junction would be fine
and I don't think any demolition would be required. The Marylebone Line
would be expensively interfered with as the new line went under it or
budged it apart and came up in the middle. The Suds and Northolt Park
would become Overground only with a train from Euston every twenty
minutes terminating at a new platform at South Ruislip. The existing
Marylebone trains which semi-randomly call at the stations would cease
to call there and would give an increased service at Wembley Stadium and
South Ruislip instead.

The doubling of frequency of Overground service from Euston to Wembley
Central would probably mean the end of Stonebridge Park terminators on
the Bakerloo, leaving an off-peak service of 6tph LU from Harrow and
Wealdstone, 3tph LO from Watford and 3tph LO joining at Wembley Central.