WHEN THE FUCK IS THIS SHOW COMING BACK?!
It's kinda hard to do when the author hasn't even conceived of the final villain, because they have no idea what the final arc is going to be.
Contrast that to, say, One Piece. Where, it may have taken a while to introduce Blackbeard(or ages to introduce Imu), but you get the impression that Oda always intended for(at least one of them) to be there at the end.