Having a real designer...
It should be great to have a designer which supports all mapping (so which allow us to realize TPC, Horizontal Entity Splitting).
It should be great to have a designer that supports partial model views as I did (http://msmvps.com/blogs/matthieu/archive/2009/04/07/edm-designer-new-version.aspx, http://msmvps.com/blogs/matthieu/archive/2009/04/11/edm-designer-new-feature.aspx)
It should be great to be able to see complex types on the designer and not only on Model Browser window as I did (http://msmvps.com/blogs/matthieu/archive/2009/01/07/edm-designer.aspx).
It should be great to not lose all our ssdl changes when we update the model from db.
The designer should support editing of the entire EDMX in a way that is logical and consistent with its intent.