8. The Kovarian Chapter
Series 6 is arguably the most complex series of New Who, and leaves a fair few questions and plot holes. By the end of the Wedding of River Song, the major plot point of the series is wrapped up and besides a throw away line in the Time of the Doctor, the series main antagonist, Madam Kovarian is never mentioned again. However, the villain does make an audio return.
The Time of the Doctor reveals that basically the Kovarian chapter were a very extreme branch of the church of the Papal Mainframe, seeking to prevent the centuries of conflict that occurs on Trenzalore. Kovarian and her followers travel back through time to blow up the Doctor's TARDIS so he cannot reach Trenzalore, but this creates the cracks in time, destroys the universe, which forces the Doctor to fix the damage. Since her first plot fails, Kovarian's second plan is engineering a psychopath to kill the Doctor.
Big Finish's third Diary of River Song series picks up with the character and what she does next. At Demon's Run, Kovarian had additional assassins, also 'Proto-Time Lords', created from Melody Pond's (River Song) DNA, and orchestrates a second attempt on the Doctor's life with these new agents, but this time, the plot revolved around the Fifth Doctor.
Of course this is another unsuccessful attempt on the Doctor's life, but the story is a terrific expansion of Series 6 and the characters of River and Kovarian.