Kamelot and someone else with good, clean vocals. Or, perhaps go the complete opposite and get someone like In Flames to headline it (need a big band that'll bring big crowds).
That would be right up their alley judging by the new album.
Mercenary are pretty cool, I'd like them a lot more if they let Mikkel Sandager (correct me if I'm wrong, but he's the guy that doesn't do the death metal growls, right) handle all the vocals.
I dunno about that ever happenning.... too much alcahol comsumption from both bands might crap things out! The bar tabs would set the tour back finacialy.
In light of that... I think it would be fun as hell to hit the U.S. with these fine Swedes!