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i was just imagining how cool it would be if we could have clips overlapping in the timeline view mode of The Project tab...

And its not that crazy too, we already have the option of multiple audiotracks there.
It would dramatically increase possibilities and in som cases completely change how you could set up projects
but off course.... to implement this would turn also pretty complex fast and would be asking lots of additional improvements and added functionality of the project view possibilities.
like for instance non destructive fading/transistions...
clip opacity... scaling... blendmodes?

I think this is already covered by my (and any other) video editing software (which handles still images as well). If I should need that, I'd use that.