A contract is a condition for payout of the other items in a suggested trade, and such a trade would be delayed until the condition is met. Any cash involved needs to be "frozen", not available for either party until the contract is fulfilled or violated. Of course any contract would be void if the involved parties declare war on eachother. It will allow a "mercenary" vocation (like the arms dealer you can currently play) as well as increase the range of diplomatic options.
Different kinds of contracts could be:
* Destroy X supply points worth of military (or any) ships belonging to a given civ - also counted as fulfilled if the civ in question is eliminated
* Destroy X freighters belonging to a given civ - also counted as fulfilled if the civ in question is eliminated or have no freighters at any point
* Destroy or otherwise remove a named set of units from a given civ's control (Influence starbase 932 which is menacing your fringe colonies, for instance) - also counted as fulfilled if the units are bought or handed over through intimidation from the civ in question to any other civ - void if the unit(s) are scrapped (haven't noticed the AI ever scrapping anything though).
* "Liberate" a given planet - fulfilled when either party involved in the trade controls the planet or the colony is destroyed
* Maintain war with a given civ for X weeks - violated if the bound party makes peace with the civ
* Maintain peace with a given civ for X weeks - violated if the bound party gets involved in a war with the civ
* Respecting territory/influence borders for X weeks - violated if the bound party enters your influence zone or within Y parsecs of one of your colonies with a military ship (unless it's at one of the bound party's colonies or starbases of course). Ships currently inside forbidden areas cannot be parked and can only be set to autopilot somewhere outside of them, unless you're willing to violate the contract (forbidden areas should be marked so the player doesn't inadvertently break the agreement).
The last two points should make it possible for the AI to give in to threats from the player also at higher difficulty levels. I'm guessing it currently refuses to bow to threats because there is no guarantee that the player won't just take the tribute and immediately attack anyway. Even if you're at the door, they're busy with several other wars and their military is obviously incapable of defeating you... This way, the player is still free to attack, but will have to choose between receiving the fruits of extortion or satisfying the lust for conquest. Getting the AI to correctly gauge the value of all the different offers outlined above is sure to be tricky, but I'm hoping the options will come along eventually.
A few less embellished suggestions I'd also like to see implemented:
* Diplomacy option for paying a non-time-limited tribute of X bc per week in exchange for peace, breakable at any time by declaring war.
* Military access, allowing ships to refuel from the other's colonies and starbases. Means less "I don't care if you go to war with me, you're in the other end of the galaxy and are no threat".
* Consideration of the results of the last few fleet confrontations when deciding who is the stronger part, for calculation of whether peace is desirable. I'm currently playing on crippling difficulty. I'm trashing Arcean fleets with minimal losses, I've taken over half their planets and am laying siege around the effect of their powerful military starbases leaving them no chance to attack me, and they still gloat as if it was the other way around. It's okay that they refuse to make peace, but I'd like the AI to have a clue that it is being torn up so it can request aid from its friends.
* Generally lower threshold for ganging up against the perceived strongest civ, particularly on higher difficulty levels, and greater cooperation when doing so. Currently they're only talking about covert action, when the underdogs should be sharing their research resources, coordinating their efforts so no two are researching the same, gather their fleets for assault and generally give the top dog as much grief as possible. "This calls for immediate discussion!"
