Recently I came upon the Sandbox for Proposed Questions of Worldbuilding SE, and it seems like a good idea.
I thought of this also because (as some of you might have noticed) I've played posting some general interest questions with answer at favourable times to get them out in the Hot Network Questions list. I got good success with one and modest success in other. I think such questions can be a good tool to attract more people to the Earth Sciences SE community, and they can be more successful if they are drafted with the help of the community in a sandbox like the one they have in Worldbuilding. Some of these "promotional" Q&A can be community work and posted as community wikis as well.
Of course such sandbox could be useful for any question, not only "promotional" ones.