6. Fenric
Fenric proves a very interesting proposition in Doctor Who lore an immeasurably powerful evil force named from Scandinavian folklore yet drawn from Zoroastrain mythology about dual beings governing the world, he(?)would certainly offer a ferocious opponent for the Timelord. Within the canon, he proves to be so formidable an enemy that he has to be trapped in order to keep him subdued. Yet this all seems rather formulaic, and done a million times befre. After all, unlimited power does not an interesting villain make, and what marks Fenric out as interesting is that he can control peoples destinies, making it so even when they acted seemingly autonomously, theyre actually being manipulated. So think of him like The Beast, except far less blatant when hes taking hold of people. If theres a better way of setting up for an epic season finale than an enemy treating you like a puppet unknowingly, I dont know what is. Fenric controlled Ace before, so to see him control Clara or any unspecified companion would be an interesting proposition.