It's extremely difficult to make a Doctor Who villain that has some sort of lasting impact. After 13 Doctors, 58 years, and hundreds upon hundreds of stories, it's really only the Daleks, the Cybermen, and the Weeping Angels who've entered that upper echelon, becoming all-time greats that will probably stick around forever.
Most villains - no matter how good they are - show up once or twice, get dunked on by the Doctor, and then never return, fading into the endless void that is Doctor Who canon. With this in mind, the Whoniverse is littered with villains who are deserving of a comeback at some point in the future.
Some have interesting stories that are just begging to be explored, some were treated poorly the first time around and need to be given their redemption, and others - particularly those from the classic era - would benefit greatly from modern special effects technology.
With a few returning villains already on the docket for Series 13, there might even be more that we don't yet know about - and these are the ones we'd most like to see.
10. The Sycorax
After waking up from a rather long nap, the Tenth Doctor's first adventure saw him successfully defend Earth from a hostile race of aliens called the Sycorax.
While their skeleton-esque faces made for a terrific bit of creature design, they weren't exactly the most memorable of villains, but all the same, a return for the Sycorax would be most welcome - mainly due to where their story last left off.
To recap: the Doctor battles the Sycorax leader in a sword fight, beats him, and then absolutely humiliates him by killing him with a precisely-thrown satsuma. The rest of the Sycorax then attempt to leave planet Earth, but Harriet Jones has other ideas - in a cold move, she orders their ship to be destroyed instead.
Crucially though, that was just one tribe of Sycorax. There are plenty more of them out there in the universe (as evidenced by their cameos in The End Of Time and The Magician's Apprentice), so wouldn't it be fascinating to see them return to Earth in a revenge story, forcing the Doctor to deal with the consequences of Harriet's actions?
The 2008 comic-book story The Widow's Curse even explored a plot like this, with the wives of the dead Sycorax tribe bumping into the Tenth Doctor and Donna. Sure, we don't get a lot of "sequel episodes" in Doctor Who, but we should - and this one would be great to see.