Migration is a great feature.
To avoid one kingdom dropping cities and having all strong players in I would suggest migration allowed to cities 3rd in power or less.
It would make dropping cities more difficult to absorb players and it would make 3 competing kingdoms.
Hopefully 2 kingdoms with less power can make alliance against stronger one so there is some balance in power.
Bipolar order leads to domination of one kingdom very fast. In that case server dies after one kingdom dominates.
3 kingdoms make big difference especially with new alliance feature.