I think London may be one of the biggest cities without a football club in its name. If one of the smaller clubs changed its name to 'London Utd/Town' they could guarantee themselves a future generation of fans.
There are many supporters of northern clubs (Liverpool, Man Utd etc) in London. I'm surprised these clubs don't start their own 'cinemas' in London where fans can go to see the home games live.
On a similar note I think someone could make a replica of the Cavern Club in London where tribute bands to the Beatles and other 60s group would play.