Sorry, I have to write not tablets, but consumer-only devices.
Recently as an experiment I tried to use Nexus 7 to work with a few hundred of images and videos to see if the interface allows that (the hardware definitely does). It turned out with a default setup one cannot do that. Just opening a file was pain. To mitigate that I installed a file manager, and split the collection into folders based on a time stamp. Then it became manageable to the point of allowing to create a movie with several videos and slide-shows. However, as video editors for Android that I tried do not permit to insert all images from folder, creation slide shows was very time-consuming.
The bottom line is that I do not see how one can avoid hierarchy when composing documents or projects from other, potentially numerous, documents. And its a pity that the current development assumes that a touch interface is only for consumption and occasional simple modification of a single file rather than trying to explore possibilities for content creation/aggregation with it. There are things that one just cannot do efficiently with mouse/keyboard that touch in principle allows.