If a serious foreign policy crisis emerges, he could be a dark horse veep like he was in 2008 for Obama and 2000 for Gore. Heck, he could be a backup if Clinton is indicted and if Democratic establishment officials are worried about Sanders. He won't add anything electorally other than NH.
Kerry was not seriously considered for Obama's VP at any point. Although I suppose you did say "dark horse" but that's a cop out by which you can claim every politician ever was a dark horse because someone said their name on the internet.
