Here are a few bands that I've never seen live, that would be awesome at PPUSA:
Blind Guardian: I've never seen them before, and it would be pretty cool to see them in that environment, perhaps doing Nightfall in Middle Earth in its entirety.
Saga: I don't know if they would necessarily fit, as they're more hard rock than metal, but they are a pretty fun band based on the bootlegs I've heard from them over the years.
Eternity X: Given that the band broke up in the early 2000's, this is very unlikely, but it would be cool to see them in one of the mid-day slots.
Vanishing Point: I am very impressed with their recent CD, and I would love to see what they can do live.
Demons and Wizards: If anyone could get Hani and Jon to come together for a US show, it would be Glenn. Watching those two on the same stage might be the high-point of my concert-going life.
U.D.O./Accept: I've not seen either band live, so either one of these would be awesome at center stage.
Doro: Due to medical reasons, I missed out the last time she played Atlanta, and I've always regretted that.
Eden's Curse: I've been very impressed by their work, and they'd fit in as a good mid-day band.