This is a great map of what the system will look like in the future. Coming to this a bit late, and reading through all the various posts, one thing that hasn't been considered/commented on is potential closures between now and 2040.
Could any of the network actually close between now and 2040? the redevelopment of Heathrow could result in T4 being closed whilst the dangelway (cable car) is one of those vanity projects that may not have a long term future especially when current equipment becomes life expired and in need of expensive upgrade/replacement...
I see there has been much talk about the proposed splitting of the Northern line and possible renaming of one or both of the branches. personally I like the idea of the Charing X branch being renamed the Hampstead Line