The “crime charged” for which he pled guilty was never a “crime”. Hence his guilty plea is void.
He pled guilty to lying to the FBI. Under the circumstances as we now know them, I.e., that there was no legitimate predicate for the interview, then the charged crime was never a legitimate crime.
It was the FBI that committed the crime.
Flynn has been totally exonerated. Whatever the judge does at this point will be void. He cannot bring in a third party to prosecute what the Justice Department has determined cannot be prosecuted.
The problem here is that it seems the DOJ hasn't provided sufficient information in its filings to reinforce the idea that the DOJ was guilty of wrongdoing in the case. If anything, I'd say there's a good chance they want to drop the case to AVOID making any admissions that should really be brought into the public domain.