For me, the killer feature would be the ability to specify a SQL database to connect with.
I (like many others who use this software) have a MySQL server up and running for my various blogs and project software, accessible through the internet (ie mysql.erikswebsite.com). It would be great to be able to set up a database on the server for Klok, and then simply point my instance of Klok to the server, along with login details and database name. This would then allow me to access my data from any of the many computers I work on, and provide for easy automated backups along with the rest of my databases. Also, it would enable the user to hack together integration with their own website or other services. It would be really easy to throw together a PHP file that could handle the above suggestion of allowing interactions such as: http://www.erikswebsite.com/klok.php?add=“My new task”&start=20090217-12:34&finish=….. etc