... or slightly more accurately, that I don't know how to write =)
I think it would be a cool format for a blog to have a title and a subtitle for every single Post. You could easily do it with Custom Fields, but this plugin would alter the Admin screen for writing posts to insert an additional text area underneath the title and above the content area.
Then there would be a special function for displaying the subtitle like:
<?php the_subtitle(); ?>
Rich Text Titles
Speaking of titles, it would be cool if we were able to use HTML tags in titles sometimes. For example, just being able to add <em> tags to a word in the title would be cool, but that causes problems currently. It might display OK on your site, but it may cause validation problems (since the tags will show up inside title attributes and such) and may come across screwed up in feed readers. This plugin would alter the the_title() function to automatically strip the HTML tags, but create a new function like the_title_html() that would retain the tags for display anywhere you want to make sure they persist. This would ensure the RSS feed remains unscathed.
Ever look through your list of plugins and forget just exactly what one of them does? I know they have descriptions next to them, but that doesn't always speak to exactly what you are using it for and why. This plugin would just put a text field in each plugin field you could type some notes in there, theoretically to keep information about why and how you are using this plugin.
You'd probably have to also add buttons/links for adding & removing notes.
Notify All Admins
On this blog we have two admins: me and Jeff. When I write and publish a post, and someone comments, I get an email about it (but Jeff doesn't). When Jeff writes and publishes a post, and someone comments, he gets an email about it (but I don't). Since this is both of our sites, I wouldn't mind getting ALL comment notifications. But I'm not sure if Jeff feels the same way. So this plugin would just add an extra user option (only active for full site admins):
There is a pretty robust plugin called WP-Typography that covers widows in post titles. I give it props, but my personal preference is to handle 90% of what it does on my own. What I can't do on my own is add non-breaking spaces to the last two words in a post title (one of the things it does). I currently do this with jQuery on some sites which is OK, but would be better done with PHP so it happens before the page renders no matter what.
So if there are already plugins out there that do this stuff, I apologize. I admit I didn't launch a giant manhunt to find existing plugins that did these things.