Postnuke as I see it now is pretty function-centered. That means, if I have e.g. articles, pictures, and links belonging to the same topic, they are in totally different areas of the site.
More practical would be a content-centered approach: group the items by logical connections instead of functions. This reflects the thinking of an average non-programmer much better.
Here's the idea: why not introduce a hierarchical system where every item has a globally valid "parent" id that places it in the hierarchy. At the same time, finally realize the idea of center blocks, so that different kinds of contents (links, articles, etc) could be displayed on the same page.
Modules would probably need to provide a kind of "hook", which instead of displaying all (say) web links, would only return a formatted block for all items under a given parent id. The front page naturally would only contain items with parent id=0.