10 Doctor Who Villains Who SHOULD Return
6. The Headless Monks
The Headless Monks are badass in so many ways. For one thing, they're literally headless, and it's actually a crime - punishable by death - to lower one of their hoods. They're mysterious and intimidating, and nobody wants to mess with them.
They can also fire bolts of energy from their hands, useful for attacking targets at a distance. And just for added cool points, they also wield energy swords, which, let's be honest, were clearly designed to resemble lightsabers. And they look awesome.
We've seen the Monks in action just a single time, in Series 6's A Good Man Goes To War. Here, they massacred a bunch of friendly Silurians, killed Lorna Bucket, killed Strax (temporarily), and beheaded Dorium Maldovar, proving themselves to be dangerous adversaries. And while they were allied with Madame Kovarian in this story, it's clear that the Monks have their own autonomy, so they're definitely free to return as an entity all by themselves.
In Series 5, the Eleventh Doctor mentions that the Delirium Archive is "the final resting place" of the Headless Monks, so perhaps a future story could show the Doctor encountering the Monks much later in their timeline, detailing what led them to their final end in the Delirium Archive.