Separate the two functions of frames, the first which determines the positioning of elements in a viewable window, the second of which determines which elements are in a frame.
Then when a path node is set to a frame, in addition to centering the frame in the viewable window, any elements not selected for the frame should fade to invisible.
There are many creative things you could do with this functionality. Just one is that you could create nice animations where successive elements appear without any viewport motion at all (like standard animation in msppt). But when combined with some motion, this feature opens up lots of possibilities.