Optical flow computation did a lot of progress recently, and gives good estimates even in challenging situations.
I wouldn’t make an OpenFX plugin for that, because every year there’s a newer/better method to solve that problem.
I think it would be better to work on offline processing of the input sequences, which would produce EXRs containing the forward (to the next image) and backward (to the previous image) OF.
This solves the question “how to compute the optical flow”.
But then, what would you use OF for? There are multiple possible uses for it.
One you have the forward and backward OF, it’s possible to do retiming for example. I think Natron has almost everything, but it would be probably more convenient to make a plugin that does the backward warp to the previous and next frames in one pass. What would a retiming OFX plugin look like?
What other uses do you see for OF?