I think particularly on medium a niche is completely unnecessary. Medium has lots of features that ensure you can write about whatever you want and still find an audience (tags, publications, featured stories, curation).

Also, writing about what interests you is in my opinion exactly what you should be doing. After all, in the end, what you want is to be discovered by *your* audience--a group of people who share your interests and love your writing. For that you have to write about what interests *you*. If you write about something else, your audience just won't find you!

Thanks for reading :-)