Marketing means effectively reaching the largest number of people, so:
Pick A Subject Of Interest To Numerous Others
Has the subject(s) of your topic map have been in the news for a week or more over the last year? Or within a particular domain of business or technical activity?
Pick A Widely Used Language
Internet World Users by Language: Top Ten Languages.
Top 30 Languages Spoken in the World by Native Speakers
Consider technology access plus number of speakers.
Use Imaginative Interfaces
Topic map interfaces should not scream I am a topic map!
Does your interface meet a user’s needs?