ServerArchitect is a work-in-progress plugin which aims at making your server fully customizable. It right now can change how your chat looks like, a whole new permissions system that actually manage permissions (how cool is that?!).
Some of the features that are planned to be added are customizable sounds (hit sounds, bow shot sounds, death sounds), music made with noteblocks playing in the background (or foreground, up to you), custom scoreboards, staff moderation tools and much more.
Currently this plugin has these features to offer:
How the progress of creation of planned features is going: