When Opeth toured with Porcupine tree in support of Damnation, PTree really brought the house down, and made the Opeth set seem extremely boring, since they played only Damnation. I've seen katatonia live, they're excellent but Scar Symmetry opened for them along with Insomnium and destroyed them. The point is, you can't have a band that will be hard to follow. I'd love to see
Cynic
Solution .45
Porcupine Tree (since opeth will play heavy tracks)
Protest the Hero
Anathema
But you know it will probably someone they are label mates with, like High on Fail during the last tour. As for Agalloch, they deserve to tour America with a decent support band, NOT Opeth, I say Agalloch and Katatonia.