If Slayer wants to go to Europe with Gary Holt, I doubt Exodus will play Mayhem. I guess it's possible that they could get Rick Hunolt to fill in again, but I really doubt that he can take another full summer off from work or whatever takes up his time these days.
Brocas Helm/Hatchet/Exmortus/Midnight Chaser 11/22
High on Fire/Kvelertak 12/12
Exodus/Nails/Hellfire (???) 12/20
Neurosis/Bl'ast!/YOB (???) 12/29