General Motivation for a general audience sounds like a hard focus point. Try breaking down- all motivation have singular aspects outside of faith and religion. What are your topics? Relationship, Career, depression/Mental illness, etc?
Sometimes there's a need to talk things through to fully understand what it is you have rather than switching focus. I've been to art school, I've proofread and guided lots of writers through this- the creative process takes some analysis, but sometimes you're just too close to see what's right in front of you. Take a step back, do some general blogging. Tumblr is great for becoming engrossed in various cultures- the motivation culture is one of them! Take time to know the general community and that will help.

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!

You don't have to pick a niche do what you're good at and what you feel spiritually called to that brings you happiness and joy that's the way I gauge whether I'm on the right path or not and whether what I want to do is in alignment or not. And as long as it's something that's serving allows you to use your gifts to serve the collective to uplift the vibration bring more hope and love and joy then you're on the right path and use
whatever Avenue works for you that excite you brings you passion but as well as helps you get out of your comfort zone Learn and Grow all at the same time