Matt Blunt would be a solid contender. Is he considering it?
Assuming his dad wins this year, I'm sure a lot of voters would be skeptical of having a father/son pair
Yea, having two Senator Blunts would be unpopular.
Roy Blunt (2011-)
Matt Blunt (2019-)
I think Hanaway or Steelman could beat McCaskill. Maybe Wagner as well. However, if Hillary Clinton wins, McCaskill may not want to lose reelection in 2018 under an President Hillary Clinton. She may want to be considered for a cabinet position in a Clinton/Kaine administration.
If Clinton nominates McCaskill for a position (Attorney General, Supreme Court, etc.), she could resign her seat and let outgoing Gov. Jay Nixon choose a replacement (maybe Nicole Galloway, the State Auditor, or Robin Carnahan, the former Missouri Secretary of State. Galloway and Carnahan may still be vulnerable as freshmen senators heading into 2018.