How about this:
the possible topics coincide with the major pages
listed on [[Main Page]] (from "Astronomy" to "Visual Arts"). The
shortest link path from such a topic page to an article defines that
article's topic. If there is no such path, then the article is
classified as a topic orphan.
An alternative idea:
For any page follow all the links from it down to about 3-4 levels,
and assume these are all on related topics. To make this more
accurate we could follow only two way links. Then strip out any
article which has more then say 50 double links as it's likely to be
the front page, or something similar unrealted to the topic.
I think that this would be more problematical than using "what links
here". The links on a page include ones to years and countries where
the discussion usually has nothing to do with our subject of interest.
"What links here" had more specific reason to link to our subject.
Eclecticology