So, what do you believe has to be added to the finished work of Christ to complete His incomplete act of propitiation?
I don't believe anything has been added.
For example (maybe as an explanation):
I believe that we fix our hope on the living God, who is the Savior of all men, especially of believers. Does this mean that all are saved? No, because it is speaking of God and not men. Jesus is the Savior of all men, especially of believers.
God loved the world in this way, that He sent His Son that all who believe will have everlasting life. Does this mean that God loved the world as a whole (humanity)? Yes. The way He loved the world, however, is that He sent His Son that all who believe will be saved.
Jesus is the propitiation for all human sin. Does this mean that all experience a propitiation of their sins? No. The subject is Christ and not men or human sin. Jesus is the Lamb who takes away the sins of the world. Jesus is the Way, the Truth and the Life, and no man comes to the Father but by Him.
As Jesus said on the Cross - IT IS FINISHED. And our hope is in Christ as God vindicated His Son by raising Him from the dead. Nothing to be added. Christ is the "last Adam".