I think he's a new character. It would be the most interesting option to me. I've said it before, I'll say it again - why would I want to see a story I've seen before retold with different actors? Why would they go to the trouble of rebooting an entire universe to retell a story that's already been told?
Bringing back a character doesn't mean retelling the same story that featured him. The Wrath of Khan
was a completely different story from "Space Seed." The 2009 movie had Pike, but it was completely unlike "The Cage." It also had Sarek and Amanda but was completely unlike "Journey to Babel" (though parts of it were very close to scenes from "Yesteryear").
But none of those characters were really the focus of the story. They were there as familiar parts of the setting to establish that the universe is similar and familiar and then they blew up Vulcan to show it was very different and the focus of the movie was getting the crew together. The origin story.
Now, you're not wrong - they could bring in Khan and do a different story with him. Or Mitchell. The problem is, Space Seed and WNMHGB were stellar, memorable episodes and TWOK is still a perfect movie. It doesn't make sense to compete with those - and make no mistake, they will be compared if that is the case.
I prefer to see the Star Trek mythos expanded upon. I'd rather have a new villain to hate than a different reason to hate Khan.