Possibly have monthly or every other month ooc comm posts where people can brainstorm up things to do for missions and what that would include. Part of the problem (if there's even a problem) is that not everyone uses discourse or plurk, or has everyone added on plurk. So the conversations sort of stay between a small group and it can feel a little bit like it's already set in stone and new parties aren't welcome, or that new parties don't even know about it.

I'm a big advocate of using the ooc comm for actual communication, since it's a location that everyone in the game should have added and has access to. So a 'what missions would people like/what exploration would you like to see/what questions do we need mod answers for before we progress' type of post every once in a while just to plot/plan things before they go up could help in just brainstorming something as well as having it be in an open area for people to add in their two cents or feel welcome to join in the discussion.