If you are looking for a cool plugin to write, I believe that it would be really useful to be able to message inside the WordPress admin pages. I have multiple authors and I would like to leave them messages which they would see when they log in and be able to leave comments/critiques on drafts and even as a QOTD or an MOTD for all users of the blog.
This idea could be further expanded to share messages within a certain peer group of blogs. If you have my PeerID, you could receive and read internal messages from my blog that were added to your username on my blog. This should be relatively easy to do if login names remain the same across blogs. I imagine that I would log onto my dashboard to see all the messages/comments that people have left for my login on all the blogs that I am a member of. I can see this being very useful in community type blogs and for people that co-author more than one blog.
Anyone think they can execute this?
[EDIT] For other “Developer Wanted” sort of ideas, check out the Developer Wanted Category on the pluginblog.