Lokis Chitauri army was not exactly full of character, but the sheer scale was enough excitement. What made The Avengers so great was the villain. Loki, to my mind, is up there with Joker, Bane, Doctor Octopus and General Zod as one of the great comic book movie villains. Tom Hiddleston had developed the character into something more scarred, unpredictable, vicious, relentless- a villain who struck at the heart of every single Avenger, and became hated by every fanboy in the land for killing Agent Coulson. The fact he made it all so personal, and to quote Tony Stark, Managed to successfully piss off every single one of , shows he was a villain worth teaming up to fight- he was a real threat rather than some hammy, one-dimensional bad guy. So, The Doctors need a great nemesis. Out of all The Doctors rogues gallery, who fits the bill? The obvious answer is The Daleks. But I reckon The Master is the best bet. Establishing a brand-new villain would waste time, and The Masters personal history is entwined with The Doctors, making it all a lot more interesting. And rather than trying to take over the universe (bwa-ha-haaaaa!), give him one single imperative- to destroy the Doctor from the inside out. Attack all of him at every possible angle physically, mentally and emotionally from every stage of his development. We have seen The Doctor go dark before, showing us there is very little stopping him from being like The Master. So The Master can test that and push him to his limits like never before. But we cant just have Doctor-Master interaction for two-plus hours. The Master needs and army behind him. Make it obvious and say The Daleks and The Cybermen. And leading them, Davros and The Cyber-Controller respectively. So, whos really in control? Whos deceiving who? Now that is a real threat. Weve seen Daleks VS Cybermen before in Doomsday, but to bring together The Doctors three greatest foes, and their leaders, now rarely-seen? That is a powerful, Loki-like enemy and an army that actually improves upon The Chituari.