Interaction Overview Diagram is needed!
Interaction Overview diagram is so much similar to the Activity diagram. It lets us to see the bigger picture of the project flow and shows how several interactions work together.
It uses different interaction diagrams(Sequence, Communication, Timing) and ties them together to make a single complete picture of the interactions.
If it would be added to StarUML then I can say that StarUML has all of the necessary tools to rock n roll!