Our game responds really well to memes, I've learned. Or we have a gamewide plot post, where people can go "here are my characters, here are lines I'd like filled/lines I'd like to pay more attention to" that tends to get people excited to play with each other. They can mostly handle plotting on their own over email, but making it a community activity instead of leaving it to individual people makes people feel comfortable just throwing out ideas instead of having to tailor their "pitch" to the exact person they're emailing.