A plugin system for the panel would be great as it would allow for a very streamlined panel with just 'core' functionality. This would be very beneficial in terms of efficiency and performance.
Any additional features can then be added using plugins as needed. Enhance developed plugins could possibly be installed through a repository with one click, which would make it very easy to install and update. This would also allow external developers to add functionalities which can then be installed as a plugin, and then could integrate into the panel in the correct area - eg. a plugin for email could show its options under the email tab.