I’d like to see Weird Al Yankovic or Adam Sandler. Dennis Leary would also be pretty good.
Hey, Monster 104, I was going to say Dennis Leary also. Imagine what he’d come up with for the British version’s “Superhero” and “Props” games. 
Whatever happened to Neil Ashdown? He was in a couple of shows and fit in quite nicely. Did a fairly clever routine with Greg Proops involving the Loch Ness Monster and Raiders of the Lost Ark. The only problem was, as often happens, he got typecast… became the “accent” guy in much the same way as Colin became the “dinosaur” guy for a spell. I hate it when the topics are too leading.
As far as outsiders go, I think Bob Newhart in his prime would have been awesome.
I always thought Paul Merton was the best on the British version. I’m not sure how the average American audience would respond to him, though.
Definitely agree about Mike McShane and Ryan Stiles - anyone remember Ryan doing “taxidermy”? Can’t remember when I’ve laughed so hard…