One thing we should add is the ability to add multiple styles so we can show off multiple style options and, eventually, different looks for the software, as well.
Simple for now -- multiple style sheets for customization and selection.
Ideally down the line we'll see ways to streamline style and template modification to create dynamic looks, but to start with, just the ability to have multiple styles would be a great start so sites can offer options.
More complex later -- Customizable templating system to help create dynamic looks for the various parts of the site. Index/forum list, thread list, messages, conversations, calendar, acp, usercp, profiles, all of it. Let's make Serenity truly customizable so anyone can take it and make it whatever they want it to be.