Betty should be on there for the obvious reasons.
Drew Carey would play the game well, I'd believe, and it'd be great cross-promotion, as others have said.
If it's not "below" him, I'd like to see Barker back in the celebrity chair *Barker vs. Carey would be awkward but interesting to see*.
David Caruso might bring some of the younger CSI:Miami crowd with him, and I think he also has the intellect to play the game (that is of course, based only on interviews and shows I've seen with him, you wouldn't know for sure until you saw him in action)
Jeff Probst, Phil Keoghan, and Letterman would be other obvious, cross-promotional choices that might bring in younger crowds and can also play well, but I can't say confirm the latter for sure, again.
Otherwise, I gotta give a root for Pat Sajak to get back in the Password chair. He did well in the 80's on SP, and he's mentioned before on his websites and on Wheel that's it's one of his favorite classic TV shows. He's also a good friend of Regis's, and as most of us know, fills in for him on "Live." If Pat was asked, I'm sure he would give it a go, and probably do very well. Trebek also would have the ability to play the game well, but I'm not sure how willing he'd be to do it...