Taking "confirmation" out of the picture, let's say that Stern has two choices as to make a TWD pin, or make a Rob Zombie pin. IMO, I think TWD is the wisest choice from a business perspective, but ONLY if it's the comic/tv license hybrid. Let me explain:
If TWD pin used the TV show characters and story lines within the pin, but used the artist from the comics to do ALL of the custom comic-esque playfield, cabinet and backglass art, then we might have something that will resonate with the casual player and hobbyist alike. Remember, Stern needs to cater to all possible facets of the gaming culture, from on-location ops to casual players with a garage arcade.
If you think a Rob Zombie pin will perform better on location that a TWD pin on theme alone, you're kidding yourself. And my problem with a RZ pin isn't the theme or music, I love them both, I think that the art style is too close between RZ and MET. The reason why MET is so special is because the art package is such an anomaly. Another horror-esque-rat-finkish art package from a RZ pin is just redundant and makes MET less special.
As overplayed and talked about zombies are, the first manufacturer to mass produce a zombie pin and market it well will get a lot of money on theme alone. TWD is pop culturally relevant, RZ is not at the moment.