Edge conditions in slabs and roof are as different from each other as they each are from wall ends, so the implementations would necessarily be rather different. But, like Djordje, I whole heartedly agree with the idea.
I have long wished for slabs that could include rim joists, brick, stone, etc. at their edge in section; and many have expressed their desires for more complex roof edge options.
I suspect that these features would necessarily take a fair while to implement as they would require changes in the internal definitions of these core elements (it's not as serious as it may sound - this has happened fairly often over the years) and would not be as simple as adapting the window function, as Djordje points out.
