[the Platinum Poppy]

Does anyone else think that Alan Moore eventually is going to spiral out of control like Frank Miller? Although his madness will, in that case, take a slightly different form...

First League of Extraordinary Gentlemen series: References to a number of more or less well-known figures from victorian adventure/thriller novels. A little bit of sex when they first encounter Griffin.
Second League of Extraordinary Gentlemen series: References not just to more well-known characters, but also to some way more obscure pulp stuff (in the beginning at Mars for example). At the back of each issue there's the "travellar's almanac" that starts to freak out reference-wise. And more sex, both the parts with Mina and Alan getting it on with each other, and one of the most grotesque murders in comics.
The Black Dossier: Combining references to HP Lovecroft, Michael Moorcock, James Bond and a children's book about talking dolls. Loads of sex, including an extra comic book within the big book of Big Brother era porn, we get to know that the talking dolls are having orgies, the new adventures of Fanny Hill, etc.

Now, I challenge you to imagine what a fourth League book would look like...