2. The Valeyard
This is someone who shouldnt just come back for an episode but rather have a whole arc written around him, just as he did when he appeared throughout the Colin Baker story Trial of a Timelord. While it contained four stories connected by the trial, Im counting it together as one big story as it is more connected than some of the other series long stories from the Classic Who run (such as The Black Guardian Trilogy). The Valeyard is an amalgamation of the Doctors dark aspects and while we have already seen this earlier in the list in the form of the Dream Lord, The Valeyard is different as he (for one, has a physical form) seems to be an inevitable future for the Doctor to
become him. Whether this will have changed since the Time War or the exploration of time being rewritten, of course, remains to be seen but the Doctor is getting awfully close to his twelfth incarnation which is his last before becoming the Valeyard. Imagine a series arc revolving around the Doctor attempting to alter his future while fighting an evil he seems destined to become. There might be some confusing, wibbly wobbly business but Doctor Who has never been short on that.