Unless they want to take on the WMD issue openly with the public (which they haven't wanted to so far; we'll see if Sunday brings a new strategy), they should think twice about McCain. He's a maverick, a crusader, a prickly fellow with a tendency to lecture from a moral soapbox. Plus, the Bush team screwed his ass in South Carolina in 2000. Not that McCain isn't an honorable fellow, mind you; but the first time Bush tries to stonewall anything, I can see McCain airing his beef very publically.
I want to see the truth come out as much as anyone, and I think McCain might make a good addition to the team. But don't let him run the inquiry, and don't count on him to put a GOP-approved spin on things.
The other concern is that Bush not appear to stack the commission with friendly, hawkish Democrats. Lieberman and Gephardt, both recently retired from the pursuit of Bush's job, would be excellent political choices, except that they both have been steadfastly pro-war. Bob Kerrey is a natural choice, but he's already on the 9/11 commission, and Bush doesn't want to play "my favorite Democrat" cherrypicking games. Hillary would be a great pick. (Rudy Giuliani would, too, and he's almost a Democrat.) An easy "reach-out" pick would be Bill Cohen. Despite being a (nominal) Republican, he was Clinton's SecDef, after all. Joe Biden would be a great pick. Heck, Al Gore would be a decent pick: I think he'd have to behave like an adult in this situation. Besides, the policy of regime change came about in the administration in which he served, so it's unlikely he could turn the panel into his own conspiracy theory wagon.