The payoff for who he is will have to be big. Big to fans, but big to a general audience as well. In my opinion, April's son (the guess of some folks -- not a spoiler) doesn't fit that bill. He'd be the son of an obscure character from a cartoon and a comic book. For that matter, even Robert April himself wouldn't be that big a payoff for the non-fan movie goer. And, it would give us yet another Starfleet officer gone bad. How many would that be now? Half of them? Seems like it. In ST09 Abrams deliberately tried to show heroic officers other than Kirk (Robau, Kirk's dad, and Pike).
I keep going through characters from TOS, even the good guys, and Khan is still the only one I can perceive of as having enough name recognition to be of interest to a general audience.
But that doesn't mean I'm convinced he's Khan. If he is, he's going to have one hell of a backstory that led him to becoming Harrison.
What I'm holding out hope for is Harrison's true identity is a jaw-dropping surprise. An "ohmygodreally?" moment. Khan would be OK. But would that be a surprise? Not any more. At this point, it would be anticlimactic.