A few that comes to mind for me.
Glen Close on "The Shiled" for one season was a great addition. I wish they would have been able to bring back the character in a different role, in the later seasons.
Comic-Book guy on the "Simpsons" I don't think he was around in the first few years of the show. I think he was a great addition to the cast.
Mr Furley on "Three's Company." I liked the Roper's but Furley was a upgrade because DOn Knotts is brilliant in everything he has done, just about.
Jason
Glen Close on "The Shiled" for one season was a great addition. I wish they would have been able to bring back the character in a different role, in the later seasons.
Comic-Book guy on the "Simpsons" I don't think he was around in the first few years of the show. I think he was a great addition to the cast.
Mr Furley on "Three's Company." I liked the Roper's but Furley was a upgrade because DOn Knotts is brilliant in everything he has done, just about.
Jason

Seriously, she became an almost completely different character over time, starting with that episode with the nurses ("Didja ever once think to offer me a lousy cup of coffee?").
