keyword search over XML streams

Dear all,

For some information filtering experiments I'm looking for XML data 
sources that are:

* suitable for keyword search, as RSS/Atom feeds,
   so they do not contain only numerical information
   as weather forecast or auction reports but also texts

* updated often, as RSS feeds are, so it make sense to filter them
   in real time

* but not RSS feeds, because I need something:
   - with depth deeper then 3 levels
   - where not only schema element values can be changed
     but also new schema elements (XML tags) can be introduced
     to bring new semantics to the data

Do you know any example of such data?

The application will filter such data without knowing schema in advance, 
using keyword queries.

Best,
Maciej