Kerry may be guilty of violating the Logan act. Unfortunately, it would require Sessions to bring charges so that won't happen.
Logan Act described: https://en.wikipedia.org/wiki/Logan_Act
The Logan Act was what Strzok et al. used as the basis for their investigation of Flynn. Seems an investigation of Kerry could be begun, and with better reason. His is a more clear violation; Flynn as a transition official, was not violating anything. The investigators could pop in on Kerry, question him, listen to all his phone calls, and whether there is a charge under the Logan Act, charge him with process crimes and whatever else they come up with.
That’s what a DOJ playing for our team would do, anyway.