I like this idea... doubly so if you could include it as part of a whole new Future Era (similar to the Genetic Era mod). I'd love to see the Space Race Victory be a little harder than it currently is. Anyway, maybe these suggestions could help:
What conditions should require it's construction? Techs, buildings, relations between civs...
Perhaps a Spaceport building could be implemented. In Warlords, you could even make them country-specific just for fun, even if there's no real bonus for them (ie, NASA for America, Roskosmos for Russia, JAXA for Japan... though that could cause trouble with the European countries). Then tie the Spaceport to a UN-like Vote that will allow a country to be willing to spend time\production\money in their Spaceport city, or decide against it. For example, if they vote for it, the city with the Spaceport would begin to build a project called "Space Station Component", which would be locked in the city's build queue until completion. Also, a National Wonder (with the Spaceport building as a pre-requisite) could also be implemented... the Space Shuttle Program, which would allow for +25% Space Station Component production.
I'd suggest a new tech\wonder tree as follows:
Rocketry & Satellites -> Allows the Spaceport to be built
Spaceport allows for the Apollo Program
Apollo Program allows for the ISS vote (any country that develops the Apollo program can choose to initiate the ISS vote with other countries that have Spaceports, or chose not to)
Completion of the ISS allows for another national wonder: the Mission to Mars (since, logically, we'll go there long before we ever go to Alpha Centauri... maybe tie a happiness bonus to it)
Completion of the Mission to Mars allows for Spaceship Components to be built (with currently required techs)
Alternatively, you could allow the Mission to Mars to be built directly after the Apollo Program as well, with the ISS providing a substantial production bonus (like +50%) to it. I say this because the ISS didn't necissarily HAVE to be built to get to Mars (and beyond), but it has taught us a lot about prolonged space flight and building things in space (as the Alpha Centauri Spacecraft is).
Who should be able to build it? It's unlikely that several very hostile countries are going to work together on this, so what should be the requirements for participation?
I'd say that two countries would need to have a specific, relatively high positive diplomatic relationship (For instance, an AI would only vote to join the program if they have a +5 or so attitude toward the country that initiated the vote). Have a set number of generic Space Station Components that are divided among the countries that vote to join (or you could choose individual ISS components and tie them to prerequisite techs if you're really ambitious).
What should it do? Bonuses, diplomatic changes, global modifiers
How can this be implemented?
1. Bonuses - not really sure if any bonuses could be used logically
2. Diplomatic Changes - Increased positive attitudes among the participating countries (ie, +3 = Working together has brought our countries closer!) upon completion. You could also allow for something other than the Space Station Component to be put to the top of the queue in the city with the Spaceport (using the Ctrl key as usual), allowing for production or something else, but the delay comes at a temporary cost in diplomacy (ie, -1 = Your procrastination is delaying our mutual effort!, and make it -2 or even -3 for the country that initiated the vote). If one country does that too often (especially the one that voted to build it in the first place), then another country may chose to scrap the effort altogether, leaving the burden of the remaining space station components on the remaining countries.
3. Global Modifier - It could provide a boost to science for each country that participates, which increases incrementally with each component added (say +5% per component), and maybe a set boost to the spaceport city's economy (say +10%) to account for space tourism. (I know this doesn't exactly fit with NASA or the ESA, but since private spaceports are now being built for that purpose, why not?)
The game will require some modifications either in the SDK or Python depending on how sophisticated it becomes, so what changes will need to be made to implement the idea (based on your suggestions)?
Unfortunately, I'm not good enough with SDK editing or Python programming to answer that one exactly
But, I've seen much more complicated ideas done in other mods, so hopefully I've babbled on in-depth enough to at least provide you usable ideas.
