An idea just came to me: What if Moria's current state (lack of ambition/ability, weight gain, etc.) isn't a result of his defeat at the hands of Kaidou, but rather a result of an ability that Kaidou posesses? Granted, we know Perona has a fruit that creates negativity, so that's not available to Kaidou, but what if Kaidou has a fruit that does something similar, only much worse, such as removing a person's "ambition" or "drive"? Can you imagine a person that's immune to Haki, simply because he takes your Haki away from you, literally? Considering Kaidou can mean "Church", and the other Yonkou go by nicknames, it's possible that Kaidou's name is a reference (ironically) to his ability: to take away your faith/ambition.
Just throwing this out there.