I got a plugin idea from this discussion: http://community.elgg.org/discussion/view/1595389/any-notification-plugin-that-has-minimum-scalability-issues

A plugin that would provide information on how many users have subscriped to receive different kind of notifications with different kind of notification methods. The report created by this plugin could be sent to the author of a notification plugin when the scalability starts to become a problem. This would allow the plugin author to easily get real life data about the performance of their plugin in different kind of setups.