I think Met-Al covered a good bit of info on some of these bands, but I feel like I should give an honest take on each of them as well. And I will preface this response by saying that I really enjoy some of these bands: Wintersun and Falconer are two uber favorites of mine and Grave Digger, Manowar, and Opeth are totally amazing and classic as well...but...
Wintersun - This would have been a no-brainer could it have happened six or seven years ago. Their debut still had jaws dropped and people hungry for a show stateside. Just on reaction to that first album alone I think this band would have made a solid middle of the pack band. Now, it's been 8 years since that debut was released, the new album brought the band on tour here in the States with plans for future touring meaning a festival appearance wouldn't have the same special quality that it would have had. But, who knows, with their renewed interest in playing shows I'd accept them at PPUSA gladly.
Grave Digger - amazing band, but unfortunately the vast majority of American metalheads either don't know them or just don't give a shit. I think they played a show in NYC not terribly long ago and I heard maybe 40-50 people showed up. They've got a bajillion albums and are very highly respected within the heavy and power metal scenes, but if they can't sell tickets, I doubt they'd get a nod to anchor a high slot in the lineup. But, who knows? Virgin Steele played one year, so anything's possible. But don't hold your breath.
Manowar - See above for number of albums, plus supposed respect within the scene, plus how many people actually show up their shows in the USA. These guys are fucking American and they can't get more than 100-200 a night even in big cities here. They play certain shows in Europe, and they're selling out much larger venues. A festival here would make sense, but perhaps not one that only holds 1,200. And I'm not even sure their guarantee would warrant an invite to Maryland Deathfest, the festival that supposedly shelled out 22,000 for Electric Wizard.
Falconer - Met-Al's insight to this (through his gf Orbweaver) was actually something I hadn't considered at first, but it makes sense. I love these guys. But my thought it that if they were able to play PPUSA, they would have already. I'm sure they're always on the list of bands Glenn wants to book but perhaps gets a quick no for...whatever reason. They can't do it financially, they can't fit it into their schedules, etc. They likely all work day jobs and can't get away perhaps. It sucks, but it's the way it is. I still think they'd fit very well in a fourth slot at PPUSA.
Doro - Once again: prolific, high regard within the scene, no one goes to shows in America. Would Doro sell tickets? Maybe a few. But I doubt she or her band would be the blockbuster you'd want them to be.
Cellador - Does this band even exist any more? They played PPUSA once when they had kind of a solid lineup, but I know some people who know some people and I'm not sure if Chris has a band behind him ready to play more shows. And without an album to follow up Enter Deception (An album I do enjoy), I don't know what merit they'd have to return to Center Stage.
ReVamp - Nowhere near the notoriety of After Forever, and Floor looks like she could have a future with Nightwish. I wouldn't hold my breath...but we'll see.
Beardfish - We don't get too many prog-rock bands for ProgPower, although bands with less metal in their sound like Enchant or Riverside have gone over quite well. I would welcome this band to the lineup.
Opeth - too big, and they tour all the time. That's just it. They do well when they tour the States. This is like suggesting Dream Theater play here I think. But if there's a way to get a "special" out of them, I think it'd be for them to play one of their albums from front to back. If we could secure that as a special show, it would certainly sell tickets. But I'm not really ever expecting that.
Naio Ssaion - never heard of them. Can't make any assessment of this. I would say since I've never heard of them then probably a lot of people haven't either. I looked them up and see they're European, and have two albums. And they're sort of a weird band. That intrigues me, but I can't say either way on this one.
