That's exactly it, Khan's blood regenerates. Now, it's simply the matter if it's stronger than the nano probes that the Borg uses to assimilate. My guess would be yes, it is. One of the reasons I think so is that at this early year, I don't believe the Borg were as advanced as they were in the year of TNG. Who knows, it might be during those years of Kirk and crew, the Borg was just getting started, and they are unknowingly not aware that they might not be advanced enough to handle Khan at this time. At least, I was going to have the Old Spock come to that conclusion after he warns the young Kirk and Spock of the dangers the Borg presents in the future. I was going to have the Borg look somewhat different because they are not as advanced as they are in the days of Picard, and Janeway. Of course, I haven't done the full research on it yet, which I would have done before writing a story of this magnitude. Another huge climatic scene I was playing around with was that I was going to do was that when Khan realizes that the Borg are not yet advanced enough but in the future, they will become a threat beyond even him, I was going to have him come up with a plan to cause a mass genocide and wipe out the Borg race from existence. Despite knowing how evil the Borg were to become, Kirk is not willing to allow Khan to commit mass genocide and with the help of both young and old Spock, concoct a plan to stop the Borg from assimilating Earth and put Kahn back on ice. When I write a story, I go big, and this story I was going to do was so big, it intimidated me. That's why I'm not going to attempt something like this alone. I don't mind research, I usually do a month or two of research on any fan fiction I do, but add that to the fact that I am very weak in Star Trek terminology and science... well, i just gave up on it. It's just too big for me to do alone, and I don't like doing stories without being faithful to the vision of the director of a fandom and the vision he has for that fandom or faithful to the original source material. I'm a stickler for that. If I do an Avengers story, I won't have Black Widow hooking up with Bruce Banner. It's common knowledge that through history, she's hooked up to either Captain America or Hawkeye. Bruce Banner has Betty Ross. I would try to stay as close and as faithful to the source material as possible, and trying to do that alone with Star Trek is just too daunting for me.