As a general rule of thumb, I think everything we store in include files would make sense to be exposed as a webservice.
What I found useful in the eLink System was the ability to store a query in the eLink System and query it using the tSOAP. Therefore a general system that exposes the .XQL file based queries using SOAP could be interesting for interoperability.
I could think of an index.vuh in the tsoap package, that gets a soap request with the fullquery name and the bind variables as parameters. It would be called like /tsoap/< mountpoint_for_my_weblog_package>/blog. We need this method (instead of saying: /tsoap/weblogger/www/blog) due to the fact that most queries depend on the package_id to limit the results in a sensible way.
Last but not least, everything we provide as a portlet should be available to the outside world. Finding a general way there would bring us miles ahead. Sadly I'm not sure a standard exists for including portal content (sure, we could send back the full HTML code, but what would happen with the links?).
