From and to Type Notes
Pre-An Unearthly Child Prequel. Stories set before An Unearthly Child.
Marco Polo - The Keys of Marinus Artificlal. Opened and closed by The Sorceror's Apprentice. Can't really fit anything else.
The Sensorites - The Reign of Terror Artificlal. Opened and closed by The Transit of Venus. Can't fit anything else.
The Reign of Terror - Planet of Giants Seasonal.  
Planet of Giants - The Dalek Invasion of Earth Artificial. Uncomfortable stretch.
The Dalek Invasion of Earth - The Rescue Natural. Only place for stories with the Doctor and Ian and Barbara and nobody else. Tight, but there's space here.
The Romans In-episode. There's definitely a narrative gap here, but it's been filled. Twice. It's full now.
The Romans - The Web Planet Artificial. Awkward. Ian/Barbara/Vicki stories fit better in the next one.
The Web Planet - The Crusade Natural. Comfortable fit.
The Space Museum - The Chase Natural.  
The Chase - The Time Meddler DWRO There's no gap here - Steven stows away (unseen) at the end of The Chase, and is discovered in the first scenes of The Time Meddler, but for the purposes of this reading order, we follow Ian and Barbara home first.
The Time Meddler - Galaxy 4 Seasonal. The only good spot for new Vicki/Steven stories.
The Myth Makers - The Daleks' Master Plan DWRO Again, we simply follow the companion home instead of the Doctor.
The Feast of Steven - The Destruction of Time Artificial. Though the idea of Sara Kingdom having had many more adventures with the Doctor comes from the novelisations, this gap essentially exists to allow Jean Marsh to reprise the character for Big Finish.
The Massacre of St Bartholomew's Eve - The Ark Artificial. Salvation inserts a new "first" adventure for Dodo, which both opens and closes it. No space for any further stories.
The Gunfighters - The Savages Before the final scene. An okay gap for further Dodo/Steven stories.
The Savages - The War Machines Natural.  
The War Machines - The Smugglers Seasonal.  
The Smugglers - The Tenth Planet Artificial. You can just about get away with Ten Little Aliens, but anything further would need serious narrative defending.
The Power of the Daleks - The Highlanders Artificial. Very forced. Interrupts and stops dead Dr Who's post-regenerative arc.