Kasich would make the most sense from a political standpoint, since Ohio is such a critical swing state and he'd also help in some important neighboring states. But there may be too much bad blood for Kasich to accept it (or a stubborn Trump to offer). Everyone mentions Walker, but he's at least somewhat bashed Trump before and his poll numbers have always been awful outside Wisconsin. And considering they tried to recall him I'm not sure how much he'd even help there.
Speaking of people that mentioned Christie, has anyone noticed that ever since the famous victory speech where Christie looked incredibly po'd behind Trump...Christie went awol in the Trump support stumping? They'll never admit it but it sure seemed like Christie and Trump had some sort of falling out that night. Maybe a miscommunication of endorse and support me and maybe VP, and Trump backed off that later, leading Christie to stop helping. Just a guess.