Are you used to GTK and gtkmm?
If so, you could start with the third item of this request:
When you double-click a waypoint in the Timetrack Panel (or right-click it and select “Edit”), a Waypoint Editor dialog appears. No matter what its interpolation is (Constant, Clamped, Linear, TCB, etc), that dialog always display TCB parameter fields. It only should be seen if it is set to TCB interpolation 
What do you think? Not directly Synfig, but a GTKmm stuff for starting.