I had a similar conversation with a woman I used to carpool with, fortunately for me, it was immediately after hearing the answer to the question on Relevant Radio, unfortunately for you, it was so long ago that I can barely remember the argument.
St. Paul, for one, seems to agree with you, that something is lacking in "Christ's afflictions".
"In my flesh I complete what is lacking in Christ's afflictions for the sake of his body, that is, the Church"
Col. 1, 24. NAB
Which would seem to say that St. Paul, for one, thought he could further the work of redemption by his own sufferings
- being in prisons;
- having thorns in his side;
- being captured by pirates;
- having his head chopped off
you know, St. Paul stuff. And all that is good, but it doesn't create a new Redemption
Does this mean that the Redemption achieved by Christ is not complete? No. It only means that the Redemption, accomplished through satisfactory love, remains always open to all love expressed in human suffering. In this dimension—the dimension of love—the Redemption which has already been completely accomplished is, in a certain sense, constantly being accomplished
SALVIFICI DOLORIS - St. John Paul II
And all of our sufferings add to this dimension, in a way, outside of time, as a sort of prayer in action and deed.
So that's the meaning of suffering.
The unique part about Christ's suffering on the cross was that He is God, and God doesn't do that sort of thing. So (this is what I do remember of the Relevant Radio conversation and the ensuing conversation with my old carpool buddy). Murdering God was the greatest possible evil thing humanity could do and forgiving it, using it as a sacrifice and turning it around is the greatest possible good thing God could do for us and it had to be that way.
So, if you want a heresy to paint yourself with, try Arianism. If you have the notion that Jesus was a good teacher who got Himself killed because He ticked too many people off. Then He wasn't God, or the part of Him that got killed wasn't God, either way, you're rehashing something that's been played out in the very beginning of Christendom and the only rational way around it is to consider the relationship between the One being killed and the ones doing the killing before and after the event.
Prior to Christ's death and resurrection, no hope for humanity - except in a future messiah and redeemer. After Christ's death and resurrection, humanity's place in Heaven is thrown open, the devil falls from the sky like lightning.