I'm currently thinking about "metadata in context", and how to realise this using Topic Maps concepts. It would be very elegant to be able to attach metadata inline, especially for long texts.
Here's a nice Wikipedia sample article linking to people, dates, places, and other subjects.
Exactly what I'd love to do, but these links should be more than "dumb" one-way hyperlinks. How about inline definition of association types and roles between topics?
(They actually have a page for discussing "the case for a Topic Map structure for Wikipedia".)