Nevermore is great and everything, but you can pretty much see them in the US every year. They're just not worth the PPSUA headlining slot.
I love some Kamelot, Edguy, Symphony X, Pain of Salvation, Evergrey and Angra, but they've all played PPUSA or other US dates enough recently to lessen their impact (it's how I felt about Sonata Arctica this year). If we have to go back to the well the only band I'm dying to see again is Therion (well, them and PC69 and Nightingale, but those aren't headline-caliber acts...the Showcase, however...)
We're talking headliners, people, and there just aren't that many bands who fit the bill.
Iced Earth would work (even though I've seen them pre and post-Barlow).
Gamma Ray and Helloween would work too (with tour-mates Axxis for the Showcase).
Saxon, Fates Warning, or Pretty Maids would all be great from an old school point of view. Rhapsody would work too, I suppose, but dear god they've been releasing the same album over and over for a decade now. Booking a reunited Cynic or Atheist would be very impressive, though probably too extreme for a lot of folks.