Having said all that, I think the cases that this would apply to are in the single digits, compared to the thousands and thousands of accounts being perma-banned for plagiarism. It would require a lot of time and energy from the admins/mods to set up this new system for the benefit of a tiny handful of users, and that time and energy could be much better spent elsewhere. The bottom line remains the same - if you don't want to be banned for plagiarism, then don't plagiarize.
Moderators could simply apply their discretion in the handful of cases that deserve that. I know there is the concern of setting a precedent (if we let one off the hook then everyone will want the same) but honestly most of these suggested "second chance" schemes wouldn't be any better.