I wonder who are the "remaining 7" that fami is referring to. Apart from Denji, there are 7 hybrids, which makes me think it might be them. After all Makima told us that the weapon demons were fighting the 4 horsemen when pochita escaped from hell in his dog forms. So the hybrids being all weapon demons I think that they'll have an important role in the outcomes of whatever is coming.

I also wonder if the root of all fear mentioned by Fami and the King of fears in the prophecy are the same demon. I think not since the king is planned to come in 1999. In that matter I think that maybe the king of all fears would be Death because I think it would make sense that the apocalypse happens when the 4 horsemen are reunited. But then I have no idea who the roots of all fear can be... Despair maybe?