In terms of the site itself, I would want you to be able to change your username. Some sites only allow you to do it every 6 months after you last changed it, so even if Fanpop would just do that, I'd be a lot happier with the site. And if there was an app, that'd be awesome!
The only thing I'd like to really see on Fanpop is more caps and more opportunities for you to get to talk with the admins of the site, like Dave and Cliff.