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On that thought:  how does a modeller make a good nuetral face for morphing?  Is it possible that a neutral face doesn't even look human or defined in any way?

 

It doesn't matter how neutral you try to make a base model's face, as most any characters based on it will inevitably resemble the base shape to some degree. If you try to make it too generic then you risk losing geometry that is necessary for making it as different as possible.

I think the more important issue is doing your best to make the face as human as possible. If that means picking an attractive model as reference and getting as close to it as you can then that's what you do. But attractiveness is subjective, so I think the best bet is to use reference that is commonly considered attractive - like an actress or some other celebrity. You don't want to make it too specific though, because then you risk locking in certain features that can't really be changed and you definitely dont want to do that. 

Mostly it comes down to experimenting with shapes. Just don't make it look like a blow up doll. 

That's my approach anyway.

 

~Shane