Aaron Hanly, the developer behind the widescreen Letterbox plugin for Mail.app, offers a plugin template 
for others who might be interested in writing a plugin for Mail.
The XCode template is “pretty bare-bones”, he writes, but will spare you some of the hack work and provide a helping hand to plugin novices.
He also provides a longer list of suggestions to get you started.
James Eagan, Missing Attachments plugin creator, has also written a tutorial on how to make a Mail.app plugin.
