3. An Arc-Heavy Plot
This is an episode that needs to be relatively simple. Mind-bending and long-running story arcs may work for the mainstream episodes of Doctor Who but the Anniversary Special is going to be a momentous TV event and is guaranteed to get a colossal audience. People who don't actively follow Doctor Who will be watching it because it's event television and won't be happening again. So because of that, it needs to be accessible to as many people as possible. The purpose of a Doctor Who anniversary special is to be a fun, energetic adventure that celebrates the programme as an artefact of pop culture rather than one that relies on and continues long-running story arcs. The only arc-related element of 1973's The Three Doctors was the final scene that saw the Doctor's exile on Earth end. It was a significant development but was a change to a set-up that had been part of the programme since 1970 and was re-established in that same story. Fast-forward ten years to 1983 and The Five Doctors, and it was completely free of all baggage and is still perfectly watchable regardless of how well-acquainted with Doctor Who you are. And the 30th anniversary special Dimensions In Time was an incomprehensible mess that nobody could understand, regardless of how much of a fan they were. The Anniversary Special needs to be almost completely arc free. That way it can be a standalone episode that audience members who haven't been following the past few series can enjoy without thinking "What's going on?" every couple of minutes, and that can be revisited years later without having to trawl through a whole series just to understand it.