Before you launch into a topic search, you have a broad decision to make. You can choose to be a small fish in a big pond, or a big fish in a small pond.
Small Fish in a Big Pond
There are some areas that are already covered off very well. Technology and comedy are two areas where podcasters are falling all over each other to get noticed. There's lots of interest in those areas and there are lots of listeners to go around, but there is stiff competition for those listeners.
Big Fish in a Small Pond
Picking a niche market that isn't serviced very well is a good way to get noticed. Producing a podcast on a topic like freshwater microbial diseases isn't likely to garner a huge listenership, but it's highly targetted and therefore attractive to advertisers and holds great potential to make you a star in your field.