Allow a configurable engine for hiding selected regions of text from the text area.
For example: In C/C++, hide parts of the buffer that are excluded from the current build configuration (e.g. using #ifdef ...).
This can be extended to different syntax highlight styles for different regions of text. For example, MS Developer Studio shows the text in the above example in gray, while the rest of the text is syntax highlighted. Another example: Mylyn (an eclipse plugin) colors text according to the frequency of visiting it. A function that was never visited (by browsing through the source code) is hidden; frequently visited text is colored "darker" than rarely visited text.