Trading units through diplomacy

[ec429]

Is your feature request related to a problem? Please describe. Currently, unit ownership cannot be transferred through diplomatic treaties, except inasmuch as units in a city change hands if the city itself is transferred. For the Aviation WWI scenario game I'm planning to run, I want players to be able to transfer unit ownership, especially for the USA to sell military units to the Triple Entente players.

Describe the solution you'd like A new type of treaty clause, "Give unit", which transfers the ownership of the specified unit to the other player, and rehomes it to an appropriate city of the receiving player. (Possibly the game could require that the unit is already in a city of the receiving player, and make that the unit's new homecity.) As well as a UI in the diplomacy screen to pick a unit and add it to the treaty, it would be useful also to have a button, key, or menu item in the map view to add the currently selected unit(s) to an open treaty (starting a new meeting if one is not already in progress). If the unit is already in the ally's city, that could determine which nation to treat with; otherwise the player would need to specify this.

Describe alternatives you've considered Units can change ownership through the Bribe Unit action, so it might be possible to get suitable behaviour by giving rulesets more control over the bribe cost than just the linear scaling factors they have now. However, the gold in that case is spent, rather than being delivered to the unit's original owner, so doesn't really satisfy my game design objectives for this scenario. (Possible workaround there is to set the bribe cost to zero, and let the buying player pay their invoices through diplomatic treaties that only include the gold transfer. But that requires an extra level of trust between players as compared to an atomic transaction.)

Additional context Discord discussion.

[jwrober]

This is a great idea. Thanks for opening an issue for it.