Whenever my group finds itself between campaigns, we open the floor too anyone who wants to proposing to run the next campaign. Over the years, my group and I have perfected the gardening one page pitch that tells us a lot about what the group members can expect about a given game.
Generally speaking, I like to put forward two or three campaign pitches to my group. On a good day, many of the other players will put out at least one. I start with a title, and then a tagline invokes the kind of story I want to tell which is short and pain, comedy, saga, or one shot. I also include the genre intend to use, and some keywords to suggest tone.
After that I give two to three paragraphs that describe the premise of the world and the opening scenario. As well as suggesting some of the content they may run into.
If I felt a need to give a warning over a kind of content in the game, for example: if I plan on having a lot of sex in a campaign, I would mention it here.
I follow that up with one or two paragraphs and a second section talking about my ideas as a GM, this is where I include any important information like time limits, whether or not I'll be using homebrew or module content, and any really big rules adjustments.
Finally, I list some films on the TV shows, novels, and video games that I am using as inspiration for the game.
This most recent pitch document was designed to initiate a more serious and lengthy campaign, after the goofball five session game I ran of the wasted hack over the last month and a bit.
For the record, my playes chose Null Point, so my next step is to write a Primer.