Devvy
Donor
-The Metropolitan Railway tried to extent the GNCR to Waterloo after it bought the company, but couldn't get the wayleaves from the properties it ran under IIRC. Before the Northern Heights Scheme of the 1930s there were two proposals to electrify the GNR lines that the scheme would take over. First by the GNCR and then the GNR had Dick, Kerr do a feasibility study using the 4-rail DC system so its trains could run over parts of the Circle Line.
I'm not sure that's even possible; the Northern City Line is above the Northern Line between Old Street and Moorgate, so you can't easily go down without burrowing down north of Old Street and wasting 2 existing sets of platforms (not cheap!). And at Moorgate, it's close to the surface and thus has all the problems of foundations to avoid, huge buildings above, and the mess of lines around Bank. The vaults of the BoE stretch around there as well; I'm not sure where or how far out they spread, but the BoE aren't going to want lines next door to them I wouldn't have thought.
-Then there is the Wimbledon to Sutton Line. Make that part of the District Line instead of the Southern Railway. And extend the Northern Line to a terminus with the Sutton Line.
I think that was originally meant for the Northern Line - to run on from Morden and operate down to Sutton. Can't remember why it never happened though. Would have been a hell of a ride from St Helier to Bank though.