It seems there will be no such thing as one size fits all.
But maybe there are ways to draw more people in...for instance--as TinaM said--if it's a protest song, it could be a protest song against protest songs.
I really like the idea of limiting it to three chords but requiring two time signatures.
And here's an idea no one has mentioned--choose a song that already exists and use the chord progression to write your song. Just in case we don't have enough ideas already.
That wouldn't be far outside my comfort zone, though. Once in awhile I like to find an old song in the public domain, rewrite the words to suit my purpose, and start with the chords of the song and rewrite the melody. I usually end up with some different chords though.
I vote for letting whoever oversees the challenge choose the guidelines. We're all getting an opportunity to offer our ideas and preferences.