Out of your comfort zone.
Not sure if it's been suggested or done before but I think a challenge that specifically takes the cartographer out of their comfort zone would be cool. I guess this would only work for people who have actually posted a map before (otherwise who is to know if a person is out of their comfort zone or not?) but what I am thinking is specifically doing a map in a style that is known to be either a weakness for the artist or one they haven't tried before. So basically, as I prefer to do a hand-drawn style then I myself would have to take on something like a an atlas style map or one created from extensive texture usage - something I would never normally have a go at.
I guess there are problems that need to be sorted with the idea but I believe it would be a really cool way to entice people to give something else a shot.
(I have no idea how to make a poll, btw, so forget that. :) )