Have a "status friend" list which is specially tailored to provide the loose-friendship status that Twitter offers (200,000 people follow Shaq, but I don't think he wants that many Facebook friends!)
have a desktop-level app or a easy to access way to update status (like Twitter's supposedly awesome API)
Also, it should take a tip from replies on Twitter and make it a single click to reply to wall posts. Currently, i have to click on wall-to wall and another click to post. Why isn't it easier?
The integration of Twitter status into Facebook would be hard to get right, but user-wise it could have much more impact. Also, my friendship base on Facebook is much more developed and applicable to my daily life than Twitter. If one of my friends "tweets" (facebook tweets) that there's something going on in the quad, it is more applicable to me than someone I follow responding to something that I know nothing about.