I'm watching some Bob Newhart-related videos. In this one, Newhart creator Barry Kemp discusses casting the show:

https://youtu.be/L1QDJtLrm38?si=hmHgoUy6MjmsDvwz

He says their original choice for Joanna was very good, but too close physically to Suzanne Pleshette for them to cast her, because it would seem like they were just replacing Suzanne Pleshette. Any guesses on who that might have been? My initial thoughts were about Jessica Walter or maybe Adrienne Barbeau.

Considering TV actors of the era, I can only think of Paula Prentiss and Elinor Donahue, both of whom were extremely close to Suzanne in age -- and had sitcom experience.