Here's a suggestion I'm sure will never see the light of day, but one I would love to see -- rendered source code comments.
Basically, I would like to have the option of adding commentary to my code in whatever text-based format works best for the type of comment I'm making (latex, markdown, rdoc, dot, etc.), and have those comments rendered in-line, hiding the original text behind the rendered view. The editor could have a hotkey to toggle between rendered comments and regular text comments. Some kind of 'meta'-markup could be used in the comment to indicate to the editor how a particular block should be rendered.
This is a lot like Knuth's literate programming concept, only less invasive to your coding process and without requiring you to compile your file to see the formatted commentary.
Imagine, you could have beautifully rendered diagrams, equations, and discourse right in your source file.
Comments?