It's quite certain that the Central Line on the eastern side will remain very
busy indeed, at least as far as Stratford.

If London's population continues to increase it will be essential to build
additional new tubes lines. The Hackney/Chelsea route in whatever guise it
ultimately takes is pretty much a foregone conclusion but if HS2 is built and
reaches expectations, London will need a new north/south route via Euston.
Such a route should connect with various strands of London Overground as
well as other tubes lines. My suggestion is Arnos Grove to Herne Hill.