CBS/Sony has been grooming Ken Jennings for the role. They keep upping his pay grade and giving him production credits on other projects.
I auditioned for Teen Jeopardy! as a senior in HS back in the day. It had "stages" where we started with a group in KC and then whittled down the potential contestants to a final audition in LA. Trebek took an interest in everyone. I don't think I've ever encountered another celebrity (maybe 1 former Longhorn football player) who actually wanted to "hang" with regular people.