Over the years the forum has tried various category hierarchies. eg. As a separate category on it’s own, combined with something else as a “mixed” category.
Currently the way “less common” topics are dealt with is not to have many “relatively quiet” categories but, to use Java as an example, for members to start a topic in General Web Dev or Mobile eg., and add the tag to the topic. https://www.sitepoint.com/community/tags/java
If a tag gains “critical mass” there is a chance, at the Admins discretion, of it getting its own category. And of course having “Java” in the title helps too