There are multiple paths forward to prosecute Comey. One of them is Criminal Espionage. Here is how that would work.
18 USC 793(f) gives espionage a 10 year federal prison sentence. A person does not have to leak classified material to be found guilty. The law also falls on a person who only removes national security information, even if they did not share it (this law was written before the current classification system)
Criminal Espionage only requires proof that a person entrusted with "national defense related" information remove that information from its proper place or disclose that information to an unauthorized person in either an intentional act or because of "gross negligence." The courts have defined "national defense related" information broadly. In
Gorin v. US, the court deferred to the judgment of the jury as to what was "national defense related" information.
https://supreme.justia.com/cases/federal/us/312/19/case.html
Some courts have limited Criminal Espionage information to that which is "potentially dangerous" and "closely held" information. BUT, information does not have to be classified for it to be a crime to disclose it.
Applying this to Comey -- in order for him to ever classify his memos as either "confidential" or "secret," he had to necessarily admit those memos were government proprietary information and related to national security -- the disclosure of which was "reasonably expected" to hurt national security. This is fatal to his defense.
On top of that, in order to become Director, Comey was required to sign a Classified Information NDA. Plus, he already had a long history of dealing with classified information. Plus, in his infamous Hillary press conference, he openly admitted information did not have to be marked classified. Plus, he also had a long history of prosecuting whistleblowers.
In summary, there is probable cause Comey committed Criminal Espionage when he removed his own Memos from FBI exclusive control and custody after he was fired. That was one crime. He then committed a second crime by giving them to an unauthorized person. Thus Comey committed multiple felonies.