James Avery suggests creating something new for topic-based aggregation. His idea is to allow aggregation of blogs by topic and not by person and wants new things invented for this.

I think they exist.

The item-level <category> element of RSS allows for setting a taxonomy on category items and that's sufficient to make James' idea work, if  (a) aggregators were looking for the domain attribute and allowed grouping by it and (b) blog engines would allow you to attribute your categories like that. Then, we "only" need to agree on common categories (or someone just writes some up) and were set. No need to register anywhere or with a central system. Here's an example of how the elements could look for .NET bloggers:

<category domain="urn:msdn-microsoft-com:netframework">ASP.NET</category>
<category domain="urn:msdn-microsoft-com:netframework">Enterprise Services</category>
<category domain="urn:msdn-microsoft-com:netframework">C#</category>
<category domain="urn:msdn-microsoft-com:netframework">VB.NET</category>
<category domain="urn:msdn-microsoft-com:serversystem">SQL Server</category>
<category domain="urn:msdn-microsoft-com:serversystem">BizTalk Server</category>