We all saw the scan about the dragons festival and so now we are sure they are definitley coming through that gate.
Also we now find out that there are demons as well that are coming.
Though have we seen any demons besides the ones Zeref has created?....Yes.
You see how Arcadias speaks of gods and demons? You see how he is practically called out as a demon most of all? Its practically confirmation imo. I feel they put demons up as gods that would explain that god slayer magic and why its black, Not because of Zeref but because of demons which I believe Zeref had no hand in creating but an actual race of demons much like Arcadias. Dragon slayers slay dragons. God slayers slay demons.
This is just a theory but I'm quite sure this may be the case. Give me your thoughts.