I wanted to add Post Template to the QuickEdit menu, so I inserted this at line 1239 of wp-admin/includes/template.php
<label class="inline-edit-status alignleft" for="post_template">
<span class="title"><?php _e('Post Template', 'templates-for-posts') ?></span>
<select name="post_template" id="post_template">
<option value="-1"><?php _e('- No Change -'); ?></option>
<?php page_template_dropdown() ?>
</select>
</label>
I'm not a real coder and I wasn't sure how to add Default Template to the list. So if you want to change a post back to the default single.php, you have to do that in full Edit Post mode.