It has been a while since I started to explore the possibility of re-architecturing my application to adapt plugable architecture, and as I read more about Yii, I feel this is the right move. I hope to have a Facebook like structure.
Not sure what exactly you want to do, but events are very powerful. Yii itself does only have "notify" events. Means something happens and your plugins will be able to react (eg onUserLogin). You can extend the basic event class and create something like DataEvents. Means you use the modified data of an event-handler as return value of an original method. This way a plugin is able to fully replace or extend an existing application method (eg onBeforeGenerateSitemap).