additional association property (#132)
Extend the association "rectangle" property in the UML Class Diagram editor and add an "auto" block property. Basically when you draw an association between two objects, it links the two in such a way that preserves the rectangle characteristics while avoiding other objects and keeping the position (e.g. you draw the association from the TOP of one object to the BOTTOM of the other) the same.
Also whenever an arc is drawn across another arc I would have the arc being drawn make a little curved hump at the point of intersection to make distinguishing the two arcs easier.
If you have Visual Studio Ultimate 2012, under Architecture-->Diagram-->Use Case Diagram, their drawing tool exhibits this behavior.