As you publish new content on your site and Google improves it's understanding of the relationships between topics and keywords, your website will continue to rank for new keywords each day.
Regularly reviewing these keywords will help you find new topics and sub-topics that you can create content around.
With this in mind, SanityCheck now has the 'New Keyword' report. This displays the keywords the site has started to rank for over the last 7 days. You can find this report linked off the main SanityCheck dashboard.
If you are running a site that publishes a lot of content, this may be quite a big list. Reviewing the new keywords will show up many that are already associated with current pages - but you'll also find some keywords that make sense to address in new content.
I'd suggest reviewing this report once a week, and saving new keyword ideas to the Whiteboard area.