Okay let me be more specific:
- There was an experimental attempt at having a feature for changing clips speed, but that was not finished (it was buggy somehow) and it did not do motion ramping: https://github.com/thiblahute/Pitivi/commits/speed_control

- David Jordan's work on an optical flow effect/plugin for gstreamer would technically allow creating butter-smooth slow motion... but this feature doesn't exist in pitivi *today*.

- Of course, any help in implementing this would be greatly appreciated ;)