I'll go ahead and offer up a potential set of categories...
- area 51
- game development
- theoretical cs
- home improvement
- scifi & fantasy
There's definitely some overlap, so I find myself wondering if there's a way to many-to-many this and have sites be in different categories. I'm not a UI/UX guy by any means, but would be interested to see what SE's team could come up with.
As I mentioned in a comment on the question, we'd probably want to open this topic to all of the affected sites and allow each community to define their category and contribute to the overall categorization. (For example, Electronics might say "we're not hobby, we're technical!" and they definitely have that right, as well as the right to change it in the future. We wouldn't want something as simple as the heading under which it falls in the universal page footer to be in any way contrary to the site's self-identity defined by their community and in their FAQ, potentially confusing new users as to the site's purpose.)
With the ongoing addition of more and more sites, scalability of such a solution would be important. Categories will grow large, we might want sub-categories, how to handle overlap, etc.