Questing in Mac'aree has confirmed my worst suspiction. As concept, the void is just fel with purple tones. They both want to consume everything. They extend like butter over the land and corrupt it. They are destructive forces, just like the light and the arcane are set to be constructive.
I think there should have been a bigger effort to actually separate eachother.
The next expansion will be just like Legion: "This big thing is going to destroy us all and there is no hope but the heroes save the day".
It makes 0 sense that the legion wants to stop the void by destroying every world ever because they are evil, but the legion itself is the same doing things like creating the scourge, deceiving, etc. They also want to corrupt azeroth, not destroy it.
I personally would have left the Legion to be the bad guys and I would have made the void something else.