Not so fussed about the colour of the signs, but I would like to see the line beyond of Rickmansworth running heavily upgraded with longer NR trains running at 90+ mph, ideally with OHLE.
The uplift in capacity would then relieve adjacent routes (i.e. the WCML), and enable more commuter housing through the Chilterns along the line. My preference for achieving that is cutting the Met back to Uxbridge and Watford and giving NR exclusive use of the fast lines north of HotH and beyond, but the southern end is a bit of an open question.
Even if you could expand capacity at Marylebone, it's not exactly in a convenient location, so sending passengers there is unlikely to be helpful...but you're not going to be able to expand the Met to Thameslink-levels of capacity, at least not without smashing adjacent pairs of the stations around the Circle into each other, anyway, so I'm not sure what the answer is, there.
There's plenty of fanciful crayon options, though. Most of these have been done to death in the past, but off the top of my head we have:
- Marylebone low-level, Marble Arch, Hyde Park Corner, Victoria low-level, and beyond.
- As above, but instead dive down east of Willesden Green, then tunnel under the Edgware Road to hit Edgware Road instead of Marylebone.
- Finchley Road, Euston low-level (could also take WCML suburban services?), Holborn (or TCR?), Waterloo low-level, and beyond.
- Finchley Road to the Fenchurch St. lines.
- ...and so on.