The decision to start a podcast is one thing, but developing the right concept is another. Obviously you want to talk about something you're naturally passionate about, because that passion will come through to the audience.
However, I tell people to think about a few other important aspects of the show. For one thing, you need to make sure your show can generate around 45 topics a year. This is true if you're planning to produce a weekly show. If you brainstorm and can't get past 20 topics in any way, shape or form...it might not be the right concept.
Make sure the concept lends itself to a good format (segments, interviews, round table discussion, etc) and figure out ways to showcase engaging content through it.
Finally, make sure you can easily explain what it's about and why people will want to listen every week.