In terms of the site itself, I would want u to be able to change your username. Some sites only allow u to do it every 6 months after u 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 meer badges and meer opportunities for u to get to talk with the admins of the site, like Dave and Cliff.