I have mixed feelings about this. On the one hand, it would occasionally be useful to have different emotions; on the other, I don't want it to become like Lego, where the classic smiling face is now rare. Another concern is that the more facial variations that there are, the greater the number of printed faces, which aren't as durable as molded faces.
Emoticons might be good inspiration for ways to do emotions while keeping the "feel" of the classic face. If they did four mouth types (curved up, curved down, open mouth, flat mouth) and added eyebrows as necessary, it'd be a fairly ample range.