Hey very cool.
As far as needs, well I'm not sure how I'd like it to look. But from a very generic point of view, it would be nice if it followed a similar model to how you do chart. Setup the type of diagram (swim lane, flowchart, star diagram, org chart diagram, etc...) And that object would hopefully have all the logic to create the layout. Of course depending on what type of diagram you are creating, the parameters will have different effects.
It would be cool if we could just specify the diagram types, create item instances with some kind of heirarchy, and the objects would lay themselves out according. Hopefully a good subset of available images, and the ability to replace certain object images with your own image if needed.
That's my generic though