New idea for diplomacy change:
If you have above two settlements, and total population above 450 people, and one of the settlements is not happy with your rule, they will create a new tribe that will be hostile towards you, and swich your unhappy settlement to be owned by this new tribe. This can happen only to humans types and not to muskelings.
I'm sure the unhappy status need to be advanced so only if 65% of the settlement is unhappy they will worn you that they consider leaving you, and above 80% they will automatically leave after one season.