Edguy--They haven't played the festival in over ten years. I know that their last four albums have had mixed reviews, but I think that enough people enjoy their newer material and that enough other people enjoy their earlier material to make them a welcome band in the future.
Nightingale--It's been over thirteen years since this band last played the festival, and I think that they're more revered now than they were then.
Secret Sphere--I know that they weren't the best live band when they performed at the festival over eleven years ago, but they've made some pretty big strides over the past four years, and with Michelle Luppi as their current singer, they received the kick in the ass that they need to go over very well if given another chance.
Stay metal. Never rust.
Albert