Clemens Vasters: I think they exist. [Aggregation by category]

Agree, here is a suggestion for another category domain -- the common NNTP groups should also work well: The item-level then looks like this:

 dotnet.framework.interop
 alt.religions

But it is still only a suggestion to an aggregator, how to group items. Don has some related nice ideas about Filters in News Aggregators.