BuddyPress.org

you can categorize your content.

This means that you can as site or single blog admin,

– sort the blogs by thematics (fruits, pets, music,etc related)

default BP sort all blogs by name. The plugin sort them additionnaly by theme.

– each member can asign himself to one or more thematics

default BP sort all members by name. The plugin sort them additionnaly by theme.

– create categories as well.

Here you create a category food – for the fruit theme, a category animals for the pets blogs, a category cluture for the music blogs, and so on…

And a tag system follows all your settings over the whole site. Also a few widget to display the work nicely.

Looks like a portal structure. And it is so far. Still alpha, but great possibilities for the future i think.