I think we have this kind of discussion before on another board. I'm more interested in the whole spectrum of bands than a single headliner. This year I'm going for Sabaton, Primal Fear and Matos. Yes Matos was the headliner for Friday but that wasn't important for me.
Now since it seems a headliner have to be a band of certain status, the premise was a band that have never toured before, and people were talking about prog in here (rock or metal is the same to me), I still insist The Flower Kings have that status.
I agree that The Flower Kings do have a certain status... like some former ProgPower acts, they are a long-running band that are highly respected by prog fans, but who (I think) have not done a lot of North American touring. However, while they may possibly work as a mid-show oddball band, they are not heavy enough, or big enough of a draw, to be a headliner. Rock and metal may be the same to you, but they're not for everyone... even if there is a lot of crossover between their audience and prog metal fans, that doesn't mean they can legitimately headline a metal fest.