Just listing various odd bands that could replace the uniqueness (though not the spirit) of Volbeat:
Same slot:
Clutch: Yes, really DC, why not?
Ihashn: Really varied stuff, new album coming out, easily famous enough for the slot.
Samael: Already in the states for that Amorphis tour...
Corrosion of Conformity: keeping that southerny feel Volbeat sometimes has
Lower slot:
Mindless Self Indulgence (These guys don't fit in a single bit, but they're fun as hell and extremely different. I've seen them live too, and it did not disappoint at all)
Powerglove (Video game metal!)
The Black Mages (More progressive sounding video game metal, Japanese style!)
Psychostick (Just strange)
Sleepytime Gorilla Museum (I'd actually be happy with the higher slot, but I might be the only one)
Tyr (these guys aren't so unique, but they'd be the only band wearing chain mail...)