Bill Cullen on P+. He had the format down slick but was just too slow. Apologies to fans of Bill Cullen.
Bill was brought in on PP to fill in for Allen Ludden who had surgery. I believe I read that Ludden requested him. Because he walked with a limp from childhood Polio, they had work around him there, going to commercials when they otherwise would not, etc. Clearly the PP set was not built with Cullen in mind, but he and everyone else knew it was only a temporary thing.
Now to address the original question. I've got quite a few names here. We've already mentioned Davidson and Anderson. Here's a few others - remember these guys?:
Mike Darrow - The $128,000 Question and Jackpot (revival) - replaced on both shows by Alex Trebek and Geoff Edwards, respectively
Dick Martin - Mindreaders - great on "the Cheap Show" - a bit awkward on this GT show
Jon Bauman - Hollywood Squares portion of MGHS Hour - the less said the better
Bill Cullen - The Love Experts - a Bob Stewart show and it was a paycheck for him
Rolf Benershke (sp?) - Wheel of Fortune - what did Merv see in this guy?
Patrick Wayne - Tic Tac Dough '90 - weeee looooooose
Pat Finn - Jokers Wild - better on Shop Till You Drop
Lynn Swann - TTTT '90 - brought in to replace Gordon Elliott, who had his own contractual problems
Chris Wylde - Taboo - I still have nightmares from what I saw on YouTube