With both Delain and Leaves Eye performing at the main festival, Epica brings very little to the table with regard to overall festival appeal. And I'd guess that's something that's heavily weighted.
Blind Guardian isn't likely to come over for a one-off. I'm guessing if they were available, they'd be headlining the festival.
Volbeat is an amazing live act. And I'm sure if they had played ProgPower when they were scheduled, they would made enough fans, that they'd have positioned themselves to headline the pre-party.
I wouldn't be surprised if Sabaton is Shane's safety band. They have both drawing power and would love to come back, probably even as a one off.
While those who know me will suggest the comments that follow are biased, I believe it makes them no less true. Nevermore, assuming Shane can get them and Glenn won't put the kibosh on it, is a home run. They have drawing power, they'll help sell tickets to the main festival, they satisfy a cross-section of both Power and Prog fans, and they have a fanatic enough following, that people will travel to see them. Plus, as it's been pointed out, they've performed very few U.S. headlining shows since Dead Heart.