Trump is inarguably the most polarizing figure in modern politics, so I gotta think the smart move would be a centrist, someone who the public can trust, someone who can repair the bridges he's burned on his way to the top of the polls.
The problem is that I have no idea who would even be willing to attach their name to this wild ship. It'll depend on what issue he wants to campaign on. If it's ISIS and Islamic terrorism, he'll need a foreign policy wonk, maybe a General (although there hasn't been one on a major party ticket in generations, if i'm not mistaken),
more likely a congressperson on some FP committee (have no idea who though). If it's immigration, he'll need someone who can back up his promises with concrete plans, someone with immigration chops.
As for their demographic, I feel like a woman could be an interesting move for him. A smart woman could take a lot of the wind out of Hillary's sails. The way his campaign is heading, I'm not so sure how much a minority would do for him tbh. Traditional logic would dictate a politician, but traditional logic has taken a backseat this election, so Trump could really go off the board. Honestly...I feel like Meg Whitman might be a very dangerous option for Trump