In many cases, people will be collaborating when doing animation.
It would be useful to be able to add comments to the keyframes of the animation.
Comments could be given in three categories:

1. Dialogue
2. Action
3. Notes

In this way, the director could give ideas on to the animator, the animator to the assistant, and so on.
Furthermore, comments for the frames could be used for another feature I have thought of, where you could export both storyboards and animatics from within Pencil2D, but thats another issue.
It could be in a format like the one below.