I would to suggest that if you become an indipendent kingdom you automatically get the ability to trade cities and settlements as the high level trade perk even if you haven't reached it
Hope can be interesting
I would like to suggest to insert a spy system.
A player is able to recruit characters that do covered operation or can track sensible targets for you.
If player is a mercenary can only have 1 spy
If player start a kingdom or become a vassal, can have multiple number of spies
A spy can give you...