The greatest challenge for the online publisher is choosing a compelling niche. The right niche is there, but it will require preparation and research to discover it. For example, maybe catnip is not a good niche, but matatabi is!
Interest in a particular niche is essential, but expertise in the subject is not critical. Exploring the niche will naturally increase your knowledge. However, do not over-reach: make sure you stay within your technical and scientific limits if you select a niche that demands such knowledge.
Excitement and passion for the subject will engender the same. If you aren’t excited about the niche, it will be impossible to convey any excitement to others about it. The most important thing is that you don’t get sucked into setting up a niche that is based on something transient. Avoid fads and short-lived flashes in the pan. Make sure that the niche you choose has long-term potential so that your publishing efforts don’t reach a dead end.