My experience with advertising is similar, and I have found that narrowing your topic to a specific concept or idea is much like the necessity of narrowing a target market, to facilitate reaching the people you are wanting to reach.
A good way to narrow down a topic is to ask some questions.
For example, motivation for what? why motivation? who will the motivation work for? what is the purpose? what is the outcome for motivation? what is the need in which you are trying to meet through motivation? etc.
If you would like to chat more about this, I am available to assist the brain storm~
Feel free to call me anytime!