Maybe I already suggested that - pretty sure I have - but group dynamics is another thing that could be based on personalities and add an aditional challenge. Some zoos have to exchange members of a social group, because the personalities do not get along. Sometimes this can be very frustrating for zoos, especially when the genetics would be a perfect match. However, in situations like these they are forced to change things, change breedings, change plans. In my opinion, all play modes would benefit from this aditional challenge, but franchise would most. It would make the animal market more lively, since you are not only searching for gold animals anymore, but also for a fitting personality.