It isn't supposed to come from people who haven't participated in sister sites. They're taking ideas from within the existing communities for a reason. They'r

It isn't supposed to come from people who haven't participated in sister sites. They're taking ideas from within the existing communities for a reason. They're going to take a handful (or several) from those ideas. Those sites will attract more people in those new niches. In the next round of proposals there will be a bigger pool of users to propose what the next sites should be.