2. Peri Brown
Yes, yes, I know, Peri wasn't the most popular companion, but the way she was written out of the show invites speculation as to what her post-TARDIS life was like. She is probably the only companion who has ever been betrayed to her death by the Doctor himself (who was under the influence of a mind-altering interrogation machine, but still), and her death involved becoming the new body of a sluglike creature known as a Mentor. What a way to go. However, the Doctor discovered that Peri was not killed after all; the warrior king Yrcanos, who was one of the Mentors' prisoners, managed to save her. Upon realizing that the Doctor was more than likely never returning, Peri agreed to become Yrcanos's queen and rule by his side. Personally I found this development unexpected granted, it wasn't the first time a companion was written out of the show by getting unexpectedly married (Vicki and Troilus, Leela and Andred), but what surprised me about it was how Peri hadn't appeared especially romantically interested in Yrcanos. Of course, she probably realized that the Doctor wouldn't be returning and took advantage of an opportunity to escape the Mentors and make a new life for herself. But this would lead to an intriguing reunion. The Doctor was obviously plagued with guilt upon failing to save Peri from a situation he placed her in, and Peri no doubt holds the Doctor in some animosity. Would she accept an apology? Could the Doctor even bring himself to face her?