Well, you can post it in the
Plotting Center, were you can explain your idea and find something else to RP with, or you can just open a new thread in the section where you're planning to set your adventure (Serendipity, Adela etc.) and tag it as open, so that everyone can simply post there or send you a PM to talk about it (if you want to be contacted in advance, write it at the beginning of the post, so we know we can't simply jump into the story ;) ).
Well, I hope I've been helpful, if you need anything else, just ask and feel free to PM me for anything else you could need.