Many people find that there are buttons they wish were in the visual editor but aren't there. Plugins like TinyMCE Advanced do a good job of expanding the default toolset, but don't offer easy ways to create custom buttons. Examples of custom buttons:
* <p align="right"> (instead of using style=)
* <p class="translation">
* <li class="inventory_item">
* <p class="note">
etc. Obviously there is an infinite number of possibilities. Having buttons for these things would not only be more convenient than entering them manually it would also allow really unskilled users to use these classes in their posts.
THE IDEA: Have someone skilled with the TinyMCE api write a wrapper function in PHP that easily lets plugins or the functions.php file (very relevant since classes are often tied to themes!) add buttons to the TinyMCE editor. Something like:
wp_add_tinymce_button($before_text, $after_text, $button_image);
This would probably open the door to a lot more plugins using TinyMCE buttons and would make a general "add whatever buttons you want" plugin a snap to build.