Chris Farley was cast as Shrek before his untimely death. <iframe width="480" height="360" src="https://www.youtube.com/embed/IcqJgC0WZ3o" frameborder="0" allowfullscreen></iframe>
Think his dopey sound would have worked, as it is not totally unlike the sound of Myers Shrek, but he's primarily physical dopey humor too, and that would be too similar to donkey. And you would want opposites there. So he'd likely need to play it more or less straight.
I like this better. Seems closer to a storybook tone than the edgy humor inspired by Myer's Shrek. And considering Shrek was Dreamworks' first CGI film, what a different company it would be if this was the version that came out.