For example, I really want a Rivers of London/Guardian crossover where Peter is sent to the SID as part of an exchange program, to learn about how a magical-crimes unit that's actually been functioning as a proper unit functions (and 'start dating the mysterious, distinguished, impeccably dressed magic specialist' is NOT HELPFUL, and stop laughing when I talk about how smoothly the SID functions, you don't know my life), and also obviously someone would be sent to London in his place (although I haven't decided who, because any option would be equally amazing).
Oh no, I want it! I want it very badly! I want to read about Peter trying to introduce Shen Wei to electronics, and applying community policing to the main villains!

As for who's sent to London - perhaps Guo Changcheng? I'm not sure if the Dixingren have ever been taught any English (or gotten their mandatory vaccines...) so Chu Shuzhi and maybe Da Qing are out, and Guo following the Nightingale around, dutifully taking notes, while Nightingale does his thing would be super entertaining.