I think the player, once he is established with a kingdom, should be able to offer proposals to another kingdom which they are able to either accept or reject.
Once players who enjoy the approach to play as a band of mercenaries tire of accumulating wealth/renown or the role-play elements of being a mercenary, they can experience a full-fledged diplomacy system by joining a kingdom or starting his own, unlike its current system which lacks foreign policy. Each kingdom has a reputation and suffers a penalty for declaring war on a kingdom with a high reputation - making a player's people more likely to rebel and more susceptible to bribery. Diplomatic actions include making war, requesting for peace, proposing an alliance or friendship treaty, exchanging information/secrets, requesting/demanding/offering tribute/aid as a separate function, forging trade agreements (caravans only trade with kingdoms you have a trade agreement with) and forging trade embargos.
The player can now increase relationship points through diplomatic actions.
Finally, a ranking system of all the kingdoms allows each kingdom to gauge the relative military and economic strengths of their allies and enemies, making alliances against the stronger kingdoms a natural option.