I'm liking the Andy Griffith and Larry Hagman ideas - too bad they weren't reality.
Fred Gwynne (Herman Munster) would have been terrific as a villain, as well. His imposing size would have been enough, but coupled with his voice and presence, he might have even been able to scare Columbo.
I haven't seen it in some time, but the odd thing is that Johnny Cash's character is a sort of low-key Columbo type cop. Maybe not comical, but like him in some other ways.