I find it straining glancing through massive packages with condensed volumes of text and finding the beginning and end of code blocks. In TOAD there is the possibility to enable a red line (called the Function Separator) that visually separates the blocks and something along those lines would be great to have in SQL Developer as well. It would also be nice i there could be different line colors for main versus sub programs within a code block.
There may be other ways to visualize this like some sort of ribbon coloring in the line gutter for example.
The main requirement for such a function is that the end user should be able to set the colors. Another is of course for this setting to be optional.