Add to the diplomatic options the establishment of client states out of enemies by letting them live on kind of autonomously, but having the ability to intervene in certain aspects of governance, including:

  • setting political system,
  • setting ruling party,
  • transferring population,
  • continuous resource tax,
  • enforcing diplomacy (e.g. if the overlord goes to war, so does the client state),
  • enforced improvement building and destruction,
  • etc.

Harsh conditions could lead to revolts and discontent in the client state, who may try to break free.