This is a nice idea! I checked out their project, it is under Creative Commons BY-SA is not compatible with the GPL license... http://opengameart.org/content/faq#q-ccgplcompat

https://www.gnu.org/licenses/license-list.html#ccbysa

so im afraid we can't use it?

I did some more research and couldn't find comparable quality under GPL3. I think HTML+CSS layouts are an edge case if they are implemented in a template language like twig, but understand that there's no interest in going that edge just for the email template.

Is it possible to override / extend the templates via a Plugin (like doing a partial override of the currently selected Theme without interfering with the rest of the theme plugin configuration)? plugins\CoreHome\templates\ReportRenderer is the place to be.

3rd Party plugins can have differing licenses, but I'm not sure whether a compiled template is a case of "linking" in the sense of the GPL (sounds like the java classpath exception case to me). GPL is really from a different age in this respect.

http://wordpress.org/news/2009/07/themes-are-gpl-too/ (no comment)

Someone asked a question in their tracker: mailchimp/email-blueprints#16

it would be great to reuse email templates, as long as the license is compatible with GPL.

in this case, maybe we could even kindly ask Mailchimp if the would consider to release their code under BSD-3-clause license? so that open source/libre projects like us could re-use their great work!