6. "The Daleks' Master Plan"
At twelve episodes long, "The Daleks' Master Plan" is one of the longest serials in Doctor Who history. I'd even argue that it's the longest - while "The Trial of a Time Lord" is fourteen episodes long, it's also four different stories in one, while "Master Plan" is an epic chase across space and time.
As such, there is no surprise why this story hasn't been animated sooner. 12 episodes is a huge amount to animate. So far, the story with the highest amount of animated episodes is "The Evil of the Daleks", and that's just 7 episodes. Even if the surviving episodes aren't animated - and based on the animations of the last few years, that's not likely - that would still leave 9 episodes to be animated.
On top of that, "The Daleks' Master Plan" is a story that justifies its epic length. The story features a huge number of worlds and time periods, as well as a large number of characters. It would take a large budget and a lot of time to animate this story fully. So is animation for this serial as unlikely as "Marco Polo"?
Not necessarily. The story might be extremely difficult to adapt in one go, but there is another approach that could be taken. When the story was novelized by John Peel for Target, it was released in two separate volumes.
This approach could be taken with "The Daleks' Master Plan". The first six episodes could be animated one year (possibly with "Mission to the Unknown" included), and then the following episodes could be animated the next. It wouldn't be ideal, but at this point, Doctor Who fans are used to long waits. So for now, while "The Daleks' Master Plan" isn't the likeliest story to be animated next, I wouldn't rule it out entirely just yet.