Here's a random little challenge: give London a significant canal network. Bonus points if they become a major part of the city's identity, and London becomes known as 'the Venice of northwest Europe' or somesuch. I know that the circumstances are completely different - London isn't a lagoon. Therein lies the challenge!
An idea that immediately springs to mind is the fire in 1666. Perhaps someone manages to convince the people rebuilding London to use canals as firebreaks and a water source to fight future fires? I've no idea how likely that would be, but it seems like a possible way to do it. Perhaps the fire manages to be even more devastating, resulting in a more drastic response?
I do very much like the idea though; canals are just great. Canals do also have a lot of history in the English countryside, so putting them in a city doesn't seem completely far-fetched. Don't know when they started with the whole canal thing though. A bit harder to transfer the canal technology to London if they only really started to become a major thing during the Industrial Revolution for example. Well, I guess you could just get some qualified builders from one of the other Venices of the North that are right near London, either Amsterdam of Bruges, if there are none in England at the time.