I'd love it if they did a London setting, but at the same time - speaking as someone who's been to London 13 times (coming 14) in the past three years and who wants to eventually live there at somepoint - it'd need to be done from inch-to-inch, mile-to-mile because I want a perfect replica of that city (also partly why I loved The Getaway's free-roam mode). Underground included too please.
But the one thing I worry about, speaking as someone who liked Watch Dogs and feel there's a really good core concept to build upon for the sequel, is that Chicago didn't feel like it had a personality to it. Whereas with London, it's immediately obvious, albeit up for interpretation as to what that personality is. Not to mention, IMO, Chicago felt 'too small' in the game. For a city as big as Chicago, I was expecting a lot bigger.