Last month Oracle released a statement of direction for SQL Developer (here it is). Apart from the enhanced testing capabilities the most important feature will be the support of graphical logical and physical data modeling - just like you normally would do using Oracle Designer - using the acquired CDW4ALL extension. This modeling can be used offline (file-based, using XML files) or repository based. So Oracle seems to be moving (some) essential Designer features towards SQL Developer, therefore reducing the need for Designer - which is obviously in 'maintenance mode' (no new features added).
One thing that worries me a little is that this "functionality is delivered as an optional extension". That sounds a little like $$$...., but I'm not sure about that!