Assuming the Republican nominee is either Trump or Pence, I can see John Hickenlooper or Steve Bullock giving them a really hard time. For a running mate, there is several possibilities : Sherrod Brown or Elizabeth Warren if they want a liberal anti-Wall Street appeal, John Bel Edward if they want to court conservative blue-dogs democrats, Kirsten Gillibrand if they want to run up the urban turnout. A good idea would be to have a Hispanic running mate, but I can't see anyone. Ruben Gallego is too young and inexperienced, Raul Grijalva is old.
If neither Trump nor Pence runs, I can see a John Kasich/Cory Gardner ticket, going for the "elder statesman/sensible conservative" message.
I don't think you could run Hickenlooper or Bullock against Trump, you don't want a contrast that is considered "close". But it depends on how Trump runs the country, if he does decent and people trend upwards in Approval then it's a moot point it'll be like Clinton vs. Dole. You want a polar opposite and a polar opposite will win the nomination because Trump's policies have been bad for America. If his policies work, I won't even be against him but I won't vote for him. I'll never support a man who tapped into the worst parts of his party to get elected.
