I agree with May. Although Damon claims to not have any friends, it would be nice to see an acquaintance of his from years back. It would also work to have him in a flashback- either way it would be nice to see how Damon interacts with someone he isn't trying to kill haha.
Damon would first have to feed John his blood and then kill him as you have to die with vamp blood in your system, drinking the blood while dying heals you. While it would be an interesting twist if Damon saved him, I doubt he would go to the trouble of saving him only to turn him into a vamp. I seriously doubt Damon would want to deal with John like that, or that he would do that to Elena's father.