I often use Gmail's canned responses (Labs feature) to compose new emails (not responses) so I start with a blank message and subject line.
Is there a way to get the canned response to save the subject and repopulate it when I select it?
There is not. But depending on your browser's settings for saving forms, you might be able to autocomplete your subject lines.
You can use this form to send feature suggestions for Canned Responses.
Until they fix this you could write your subject in the main body of your email, and then just cut and paste (or higlight and drag) it into the subject.
With Gmail in Firefox you can create bookmarks with "mailto" function ....
Copy text below and right-click bookmarks toolbar, click create bookmark and paste text below in "LOCATION" field. To edit, right-click and select "properties". Make changes in the same "Location" field.
Copy and paste:
mailto:"Contact Name here" <[email protected]>?subject=Enter your Subject Text here&body=Enter Your body text here
if you type !date_current_cyclops
it will replace the coding with the current date. Perfect for subject headings!