Basically, it would be really cool to have a little activity graph like the one in my signature displayed on the homepage, either in the Members Online Now or Forum Statistics boxes. These little graphs are often referred to as "Sparklines," and are used around the web for displaying data in a small and easy-to-understand. It would be really cool to see them integrated in many places within the XenForo software.
For a little more technical detail on my signature:
I'm running a cron job that parses the homepage every 15 minutes, then enters the stats (Visitors/Members/Guests online) into a database. It then reads back all the entries in the database from the last 24 hours, and uses a slightly modified PHP Sparklines implentation to render out the graph to a file.
I plan to keep this running indefinitely, which means in two years I can produce a massive graph of the activity of the XenForo forums since this date, with an accuracy of 15 minute intervals. It should be pretty cool to see.
I'm including the file here in case I change my signature in the future: