Adaptation - Describing the Doctor
In adapting Doctor Who as a cartoon, Nelvana's first challenge was to define what visual elements from Doctor Who were going to be used.
Doctor himself was a challenge. By the time of the Nelvana cartoon, there had been seven, perhaps eight (includling Cushing) to draw from, of every appearance and brand of eccentricity. Tom Baker was the most famous, of course, due to his length of tenure and PBS showings, but there had been at least two after him.
It was unlikely that Baker, or any of the others were actually going to be used. An actor's face, even in caricature, is a property, and their likenesses cannot be used, except by permission and usually by paying a license fee. Given that all of these Actors were resident in England, some were dead, some were a bit ... difficult to deal with, that seemed an unnecessary complication. Further, using the image might involve some suggestion that the owner of the face provide the voice, and further contracts and complications. If there was some likelihood that this could be done cheaply and easily, it's possible that Nelvana would have gone down this route. But given that it's transcontinental and trans-national, it wasn't likely to be cheap or easy.
So it seems most likely that Nelvana would have gone for a completely new image of the Doctor, and used a local voice actor that they had some working relationship with. That would maximize their freedom. The only requirements for the new Doctor would be that he was British and eccentric in a charming way. None of Steve Bastien's concept drawings for the Doctor involved any clear likeness to any existing Doctor.
As we can see, there were several design options, ranging from old men to young, from an extremely English 'short coated, capped' cricketeer to the more mysterious but comic trenchcoat figure, to younger or more 'heroic' figures.
We can actually dissect the sources of inspiration - there are the Doctors of course, particularly Tom Baker and Jon Pertwee known best from PBS and by far the most influential doctors in and of themselves. But by this time, through PBS almost all the Doctors would have had some exposure. But they wouldn't have had a lot of exposure to the targeted children's demographic in America. Some of those kids might be watching PBS and be Tom Baker fans, but mostly, the were tied into American media.
But there were also local influences - the reality is that the choices and concepts of a Doctor Who Cartoon were always going to be more strongly influenced by the culture of American cartoons in the 80's than by the series. Like it or not, He Man or Ghostbusters or their ilk were going to be more influential than Tom Baker. Doc Brown from Back to the Future - the model for the elderly eccentric, inventor, time traveller in his own right. The other was Egon from Ghostbusters, the younger more active version of the science hero. As much as the British Doctors, these two examples provided the poles, the two extreme visions that Nelvana had to choose between.
Which way would they go? It's like sifting tea leaves. We have the concept drawings, but we don't know who in Nelvana was involved, or which way they were going. Without anything better, I'd say, probably towards Egon.
There wasn't a lot of 'clever old men' in Saturday morning cartoons at this time. For the target demographic, it was 'Boys adventure' - He Man, GI Joe, Transformers, etc. An Old Man was a father surrogate, which definitely wouldn't be the Doctor. A quirky eccentric 'non-fatherly' old man as a hero would be hard for the 8 to 14 year old boys who were going to be the audience to engage with.
And if you went with the old guy, you'd have to have a young person to do the actual heroic stuff - Doc Brown had his Marty McFly. This had been true for the Doctor himself, Hartell had had Ian, and Troughton had had Jamie. Even as recently as the Tom Baker era, Harry Sullivan had been brought in as a companion before they'd chosen Baker, because they weren't sure what Doctor they were going to get and they thought they might need a young strong man to do the action sequences.
So the older versions were out. The demands of the series mechanics would have pushed them towards the younger, robust version.
As to the visual elements - the trenchcoat and scaff were clearly in. Lots of room for pockets in the trenchoat. We could see the Doctor often pulling strange objects from his pockets, it could become a running gag - any number of improbable or useful items could be hanging about in there.
The fob watch was a very handy symbol of time. Time references could be made constantly through it. It would have probably taken the place of the sonic screwdriver as a versatile tool - communicator, hypnosis tool, badge, flashlight or searchlight, speaker, recorder, handheld computer, tardis control, etc.
The watch chain would have allowed it to be used like Indiana Jones whip, or a cowboy's lasso, the length of fine chain could be as long as it needed to be. It was certainly handier and less bulky than Egon's backpack.
However, that doesn't mean that the elderly Doctor concept was set aside. The Doctor was a time traveller after all, and in the British series, he'd met himself several times, in fact, he'd met several versions of himself more than once. So the older Doctor was tucked away, possibly to make a guest appearance late in the series.