I've been thinking about that, here is an idea...2 challenges can go on at the same time, one has to be a grind challenge and one a roof challenge. If you succesfully complete a a grind challenge you have to post then a grind challenge, if you complete a roof challenge HAVE to post a roof challenge. See a pattern?
2 categories of pwnages, Grinds and Roofs. Each are completely separate of each other.
So basically The grind Pwnage category is up for grabs, who ever wants to post a grind pwnage may do so.