We've already speculated at length about AHS's resident Antichrist Michael Langdon. There seems to be a lot of mounting evidence that Michael will play a key role in the new season. Given the fact that he's so clearly evil at the end of season one, the most obvious scenario makes him the villain of season eight. Michael will have grown into a devilish young man, and as his powers manifest, he'll threaten the entire world.
As for the witches (who have been brought into the fold by the confirmation of the the Coven/Murder House crossover)? Well, they'll have to join forces with Michael or fight against him. We can see it going either way.