Really nice suggestion - the application for trees alone would make it invaluable, but you could apply it to overhanging rocks, clouds, birds, canopies, clothes-lines strung across a street between buildings, partial roofing, prison cage shadows, underwater surface refraction etc. You could even extend it further and have two foreground layers to allow you to have a tree < tree shadow < map setup. It opens up things from a content-creation aspect, allowing the marketplace to offer more interesting products :) It also could play nicely with global illumination by allowing all-over lighting, but with some stark noon-day shadows (which tokens would be darkened by, instead of just the background, so they can look realistically as though they are being shaded from the light). The only downside could be that each additional layer starts to eat up system resources: they probably would need to default to non-rendering unless they have content inside them. That way if you have low-end systems being used, you could opt out of using them entirely.