Maybe the challenge for the girls is to write a song for the au campaign, and turn it into a competition where the fans can chose which members song will be used and will be released as a single. Maybe it will be even more interactive, let the fans also be part of making the song.
It would both be a challenge for the girls, as they will have some direct input for a change. And also for Tsunku of course, as he has to make a song around the material that the girls and/or fans will be submitting via their au phones.
There was a TV show here in the Netherlands that did something like this. It featured a couple of producers that had to make a song from all kind of soundbites that people had submitted via the internet. So it could easily be turned into a tv show for Morning Musume.