One notable feature that we can implement better with a newer version of SPJS that supports ID would be ETA. 
It would be quite awesome to have.
Yes we need an estimator but the key on that is calculating an execution time per gcode line and then adding those up for a total estimate.
I believe most of the time is spent on move commands (and dwell commands). Computing time based on time / distance to travel in work unit would give a very good estimation. If we couple that with a Kalman filter, I think we are likely to hit the estimation within minutes.