It actually exists already, it just is not enabled by default in Ubuntu. Try OpenSuse and you will see exactly that and also a far more advanced menu system in overall...
It already exists. If you don't see it, right click on the system button, and select edit menus, under system > preferences, add a check mark to the control panel.
What advantage would a separate application that has all of the elements in the System Menu have over the System menu itself which by definition has all of the elements of the System menu?