Croup and Vandemar meet Goss and Subby
This is how I imagine the encounter of the most dangerous fictitious villains in the history of London: Neil Gaiman's Croup and Vandemar and China Miéville's Goss and Subby. Both pairs belong to different books and were created in different decades (and even centuries, to be precise), but they have much in common - enough to fantasize about them meeting each other at least once.
How did I get this idea?
Actually, it was obvious.
I think, most of the urban fantasy fans, who read Neil Geiman's "Neverwhere" and China Miéville's "Kraken", have noticed the similarities between the villains from both of the novels - Croup and Vandemar, Goss and Subby. Here and there in the internet I see opinions and sometimes discussions (luckily, not holywars) on whether it is a great coincidence or a copy.
Personally I think it is a homage. "Kraken" was published at the moment when "Neverwhere" was being considered the classic of the genre for more than a decade, so when you decide to make a novel in the setting of modern London that has its mythical side, you are completely in the loop of the comparisons it will get. And China Miéville was definitely aware of this, so it seems to me, that it was his intentional move to create Goss and Subby as a small easter egg inside a big easter egg, that is book itself.
This or that way, intentionally or not, both evil-duos have much in common, and this is a strong reason for wishing them meet. If this encounter had taken place, time-wise it would be prior to the events of either novel, because (spoiler alert) all four of them eventually are killed by "good guys", each pair in its own narrative.
So, imagine London of the 20th century, evening street with its typical houses and red phone booth, and in this exact location the encounter happens. It is not a battle or anything related, they just bumped into each other accidentally, glanced with suspicion and continued into their own directions. Both Croup-Vandemar and Goss-Subby have impressive track records in London mythical world, they are very expensive to hire and for sure both have powerful employers.
And they know that the last thing in the world they want and need - is messing with each other. So they just go.
This scene was born in my head and resulted in the above illustration, which is the only evidence of Croup and Vandemar meeting Goss and Subby. Completely fictional, but still evidence. :)