I absolutely do not think that Angra or Rhapsody could presume to headline over Stratovarius. I have a pretty good feeling that if Rhapsody are playing, they are "band #3", the 4th slot Friday night. I don't think Angra's on the bill, folks. give it up.
I agree that Ayreon/Star One or Avantasia would be big enough, but I seriously doubt that either of these things are happening.
And to the poster who seems convinced that the big overall headliner is a return act, Glenn has said nothing to indicate this. he said that there would be a third slot return act (which fell through) and that his BACKUP for the headliner was a return act, if i remember correctly. Not the headliner itself.
Accept, Demons & Wizards, Bruce Dickinson (though he apparently doesn't enjoy playing the states, from most accounts), or Rob Halford (though he's presumably very busy with the Judas Priest reunion - would fucking love to see him solo though) would all fit the bill. Running Wild possibly as well; they've at least been around a great deal longer than Stratovarius. My money's on Demons & Wizards (with Blind Guardian or, less likely, Iced Earth, as the backup band).
As far as other ideas for who it could be... Annihilator? Metal Church? both of those are very iffy as to whether they would be a big overall headliner project of Glenn's for ProgPower, though I'd love to see either.
In keeping with this solo vocalist theme: Paul Di'anno? That would be interesting - some of his recent stuff was pretty good. Though I think I read an interview with him saying there was some legal reason that he couldn't come into the country (in addition to it being unlikely that someone who has been fairly low-key since singing for Iron Maiden over 20 years ago would headline over Tolkki and co.)
Hammerfall could possibly headline over Stratovarius, though again, not likely, and i doubt Glenn would be having to work that hard to make that happen since they're just a band like everyone else, after all...
A full on Angra reunion would probably qualify, though last time I checked Hell hadn't frozen over...
I guess there's a faint possibility that Glenn is wrangling someone really old school like Deep Purple or Uriah Heep, but i also doubt that...
Oh, put me down in total disagreement that a Dream Theater side project could headline over Stratovarius. I barely even like Stratovarius but i think that's asinine. Dream Theater themselves, yes. A side project, no way.
I don't think that a Cynic reunion would ever be billed over Stratovarius, nor i do i think it would possibly happen, but if it did ever happen, i bet it would be at ProgPower

just throwin' that out there, like i always do...
Oh, and put me down as a full-fledged member of the "I would kill to see Arcturus at this show" bandwagon...
Ryan (I think that this is easily the longest post I've ever written on this forum that wasn't a review, and still i feel like I'm forgetting something.)