In the first episode of the second season of Doctor Who, the 10th Doctor was ushered into his first full-length episode; "The Christmas Invasion". In this episode we saw an alien threat that hasn't appeared on Doctor Who since.
These aliens appear to function in some form of tribal fashion with their conquering goals and customs, but they demonstrate control over earth. They are such a great alien for the Doctor to battle against. While they don't come to earth shooting lasers around, they are still a formidable foe. It is both a different and exciting take on what an alien invasion could be.
So why haven't we seen these aliens come back in another episode?
Sometimes, the Christmas Specials on Doctor Who are isolated episodes and don't tie in that much to the entire series. An exception to this was certainly made when the 11th Doctor regenerated in the Christmas Special, "The Time of the Doctor.
Even though the Sycorax appeared in a Christmas special, that doesn't necessarily mean that they cannot return in future episode. There are certainly others, along with myself, who would like to see the 12th or even the future 13th Doctor have an encounter with these aliens.
Too bad we don't have a TARDIS to jump forward and see if the BBC actually does that. Spoilers. Thanks for reading, Doctor Who fans.
Allonsy, Morley Brenenstuhl