Will My Podcast Fill a Niche?
Although this is not essential, it is not a bad idea by any means, helping you to corner the market somewhat.
Filling a certain niche may well help you find listeners more easily if there is little in the way of competition, although if you narrow your topics too much and make them a bit obscure, such as Japanese gardening on the moon, or Victorian interior design with Time Lords, you may find it difficult to find any potential audience whatsoever.
Research Your Market
Before you get too carried away with planning your podcast, research your competition.
If you are targeting a very niche market, then there may not be much of an audience waiting for you anyway, or perhaps they are all listening to another podcast, so how can you possibly compete? Ideally, by doing a better job, being more entertaining and trying just that bit harder to please and engage your audience. If what you are broadcasting would interest you, then you can be confident that it will appeal to other like-minded people.
Conversely, if you opt for a particularly vague and popular topic, then the pie will be bigger to start with, although it could still be difficult to grab yourself a few crumbs before you manage to get hold of that illusive slice.
Patience is the key to building up a good listenership, although high expectations and targets are not necessarily a bad thing, they will just give you something to aim for.