Allow super users to edit shouts. Currently, you can only edit your own shout
Shout email notifications. Some people use the Shoutbox for business purposes, such as Forex Trading. Having email notifications would be a massive benefit.
Strict blocking. Use repeatable form fields to allow admins to enter phrases that will completely prevent a shout from being submitted. This could be a good form of spam filtering.
Shoutbox design. Add 1 or 2 more layout options
Notifications. Individual users can enable/disable sound and HTML5 notifications
Banning. Use repeatable form fields for a list of banned IP's
User state. Add a small green/red icon by the user's avatar to determine whether or not they're online
...more to think of