I've just got an idea on how kdenlive could allow video composition effects in a way that would not be too hard (hopefully) to implement.
To implement video composition in a DAG fashion, as in Ramen HDR, would be too hard to implement, so I thought on how to do it more easily. My idea is: implement combining operations for 2 (only 2) videos. That simple.
The combining operations would be something like gimp's layer modes. Most of them are easily implementable. So, everything the user would have to do is: select 2 clips (or better: tracks!) and apply a combining operation. That would leave a new clip (or track) with the combination of both clips (or tracks) for the chosen combining operation.
Combining only 2 videos would not be as practical (from a user point of view) as having a DAG as in Ramen HDR, but it is a lot easier to implement and would allow the user to do anything that a DAG-based image compositor allows. Recombining is enough to achieve amazing effects.
So, kdenlive devs, what do you think of my idea?