Ok just some spitballing for additional casting for Batman 1966 characters. Two factors here are the rules. The character has to have been in a Batman comic book before or during the series airing and the actor has to be physically capable of having done the role during the airing.
I had read that William Shatner had been considered for Two Face Harvey Dent and I loved his appearance in the second animated Adam West movie in that role.
Scarecrow had been a part of Batman history since 1941 and his 25th anniversary should have included a role on the tv show. Since I like stunt casting sometimes, I think Ray Bolger as Scarecrow/Professor Crane would have been very interesting. He did the movie Daydreamer in 1966 so he was around and still active even a few years later. I think it would have been interesting to see.