well I've personally been thinking about a crossover like that for a while. If you notice that all the gods in spn are evil, maneating and so on.

So what if they're evil because their souls are stuck in human bodies over in Zealand. So what if Crowley kidnaps Odin, to get his power for himself. And the boys end up saving Axl before Crowley can sacrifice him. With the other Johnsons of course flying in to the rescue. And the prophecy of the gods regaining their power is about the NZ gods regaining their powers that are currently running around in the US without their souls.