(!) Addon to diplomacy (!)
Something like trading resources but differant and possibly hard to code:
1) What im thinking is to acctually "sell" and "buy" worker(including slaves), scouts, navy via diplomacy. What i want it to do is this:
When ever you/AI have a navy/scout/worker stationing in capitol, it can buy it as a resource (like iron) for 20 turns (10 turns is too small for worker), and "bought" worker instantly showes up in your capitol, but has no moves left, and costs no maintanance or just gives you +1 free unit, and works whith same speed as normal workers. After time runs off the trade is still active, but you no longer have this unit as a free one. When the person you bought the unit from decides the deal is done, this woker just dissapears from you country and comesback to its oner capitol.
Same thing would work whith scouts.
This was done on "slave" markets. Like Gladiators werent only thos that were cought by Romans, but also those that were bought from lets say Egypt.
For navy units its harder, as 2 condiion will have to be made in order to do this:
[a] Owner must have navy unit in its capitol, witch is also a harbor city.
Buyer needs a coastal Capitol.
2) The thing along the open border that is missing - improoving. Lets say you as a player can pay lets say 10 gold/turn, and the enemy will send some of his workers to do automated improvemants in your city exept chopping down trees and jungles, but can build all improvements the owner of this units has. For instance, you dont have animal husbandary and calender, so you cant build pasture and plantation, but if you hire improvemant works from a civ that has knows those technologies, than his worker can build those improvemants on your terrain.
Same thing should be let for you.
How ever paying cant be so easy, as you will agree to improve enemy, and you wont send a single worker to his country, but still recive the gold. This way i say it should be done this way:
[a] After you have Open Borders, you will be able to give, or take (needs permision from other side, might require war, friends, tribute, threat) the new diplomatic option "foreign works".
You wont get paid by turn, not from signing (unless signing this trade will give you a tribute of one time pay, or resource (strategic, luxuary, worker, scout, navy unit, gold in meaning of money not resource) per turn.
[c] The pay from working will be given to you when ever you will send a worker to enemy and start the improvemant (still cant pillage old ones, also meaning cant switch irigation into cottage, but can switch into mine when under a farm or cottage there is a resource that requires a mine eg copper).
The pay for work should look like this
5 gold for road (1/turn, and 2 for finish) [this may have problems whith stacked worker improving, when 2 workers build same improvemant on same title, so lets say in that case the work cost increases by 75% for every addonitional worker (adds only to base cost)]
30 gold for farm (3/turn, 9 for finish)
25 gold for mine (3/turn, 4 for finish)
30 gold for cottage (4/turn, 10 for finish)
50 gold for fort (5/turn, 10 for finish) [fort might have bad counting as i dont know how long it takes to build it] + fort can be constructed only on titles away from city radious.
20 gold for raildoad (2/turn, 10 for finish) [also might be worng calculated]
50 gold for lumbermill (4/turn, 10 for finish)
28 gold for windmill (2/turn, 10 for finish)
and so on.
3) Buying production (!)
Its reality, as Hitler bought production in russia before WW2. What i mean is in diplomatic screen, you can buy a foreign city whith opened border production allowing you to build units (only units) that will be affected (promotions and expitiance) by Buyers civics (like Feudalism, Theocracy) and wonders (Art of War), buy unit selection is limited to enemy technology and enemy resources (you can still give him/her your only Iron to make swordsman in her country), you can build your UU, but you cant create foreign UU, or foreign religion special units, unless you share same religion, but still the city in witch you bought must meet requiremants for production (access to Capitol in order to recive Iron, and cristianity, and cristian temple to build a Crusader).
Selecting the city is simple, you just select it like "selling city", so diplomatic menu should look like this:
resourcs;
-Iron
-Copper
-Ivory
-Wheat
production city;
-Madrid
-Barcelona
-Seville
production city cost:
production length
So you just select a AI city (on AI side of diplomatic screen) and tham select "production city cost" and type in the cost in hammers (gold per turn window) and "production length" witch is the numbers of units you want to build in enemy city.
By saying "production cost" i mean the cost of every 10 hammers (i prefer shields as it was in civ3, just so used to it). So depanding on city production speed is your paying. For instance - if a city has 5 hammers, and your production cost is set to 2 gold for 10 hammers, that means you will pay that foreign civ 1 gold per turn.
Unfortunatly, when the city in witch you have bought production will be captured/razed by other AI/barbarian (well, barbarians are AI too, buy saing that they are [Artificial] inteligant is a mistake ) The deal is cancelled whith no negative result on disposition, and your production as well as gold is lost.
There is also a problem whith what the civ that have sold the production. Well also simple. When you will sell production city to foreign civ, the city witch you have sold will start building "[Foreign civ name] - [unit name]" (example : Spain - Swordsman) and this build will be wealth like. If it can bring any other beneficts than money for production, it may be also be culture, and sciance, as foreign culture in in your country. Also you cannot switch this production (all other in your city are darkned[unavailable], like crusader in Cristian country whith no temple build).
I know it can be dont. Faith dies last.
*This requires opened borders, opened borders cannot be canceled, when production city is bought.*