If I had to pick one, I'd say Cory Gardner. I think he could make sense for basically all of the candidates as a young white (I doubt we'll see a ticket of two non-whites) Senator from a swing state who probably hasn't alienated any group in the GOP.
I doubt the GOP will have a pro-choice running mate (Sandoval).
Colorado has a Democratic governor, no chance.
I remind you that Colorado doesn't have any major Democrats who could be appointed. Also:
https://en.m.wikipedia.org/wiki/Colorado_SenateI think Michael Hancock might get confirmed. If he wants an easy one to pass: Happy Haynes, Bill Ritter, and Cary Kennedy would be okay.