Depends what you're doing.
Some nice lenses would be handy, if your camera has interchangeable lenses. A light panel could be handy or some actual studio lights. A filter kit. A decent tripod. A good kit bag (i recommend Kata). DOF adapter for 35mm lenses.
In fact the list goes on and it depends what you're doing. ENG stuff you won't need half of that (an excellent shotgun mic). If you're just messing around and creating mini-movies you might want cheap versions of alot of those.