3. Sycorax
We first saw the Sycorax in The Christmas Invasion and, as the name suggests, they tried to enslave the human race but lost that privilege in a sword fight with the Doctor. They then briefly returned to help lock the Doctor into the Pandorica, but as that wasnt a huge, menacing part for them, maybe its time for a comeback? How could they go about it? Well, they could try to enslave Earth again; instead of an obvious attack, they could slowly infiltrate the Earth, unnoticed until they have nearly won, making the battle much harder for the Doctor to stop them. Or, if Earth is too obvious, there are other options. The Doctor could travel to another alien planet, only to find their race fully enslaved by the Sycorax and desperate for a saviour. Could the Sycorax even be enslaved themselves? What horrible creature could tame these fearsome warriors and make them beg for mercy? What creature could show a different side to these evil aliens? Whatever it could be, it would definitely be quite the match for the Doctor.