I have one more idea : mirror & reflection... I think in this one, all people can participat, because in brice is easy when you play between surface and sky or something like. In poser and photo too, with the object and more.  In fractal, is easy with the combination mirror and symetry...
Well, I have more idea!!!

If you see good, and after the decision of the theme of challenge, i can make the translation in spanish, french and portugues (only for to see more participation ;) )