I'm sure you've heard of the Basque Country, a region of both Spain and France where the people have their own nationalist pride in the area. Now, I've no intention of introducing politics into my games (unless I'm making a political game, of course) but sometimes its unavoidable. There are several clubs in the Spanish leagues - most famously Athletic Bilbao - that are located in the Basque Country. The pride they have has generally led them to shun Spanish non-Basques, or in extreme cases refuse to play any player other than a Basque national.
Such rules are easy to incorporate into the game. I do so through the use of a list of "special" circumstances for clubs, which can be official statutes of the club. The aforementioned Athletic Bilbao will have nobody but Basque people on their playing staff, and likewise Deportivo Saprissa of Costa Rica sign only Costa Rican, both do so out of pride. Other clubs such as Sinchi FC or Albirex Niigata in the Singapore S-League play only Chinese or Japanese players respectively, a league ruling for these foreign clubs.
The special circumstances may also be more general gameworld data - a club may have significant support from a nearby country and so players from that nation may be more open to representing the club - Celtic's relationship with Ireland is a fine example that is not set in stone in any way. Celtic do not have an official policy of signing a certain number of Irish players.
All in all, it's a very data driven solution that means I can keep the code simple and add the features in the data editor. But it's this that can cause a problem. Going back to the Basque people, these people are also Spanish or French nationals. Joseba Etxeberría is a Basque player for Athletic Bilbao, but he's also got many caps for Spain. Likewise, Bixente Lizerazu played for France.
The problem is obvious - if Basque Country is in the game as one of the "nations" in the game (where "nation" indicates a non-political entity, but rather a footballing one), then how do you ensure that its people are elligible for France or Spain as well? "Second Nationality" is an available field for everybody in the game but it's not mandatory. There's no mechanism for forcing it.
The Basque Country are not alone. TCFG intends to cover as much of the world as possible, not limiting itself to FIFA affiliated nations. There are active national teams for Catalonia, for the Isle of Man, Gibraltar, Sápmi, Sark, Anglesey, Åland and Monaco. Even the Vatican City have a national football team. And crucially, players representing almost all of the above are also elligible to play for Spain, for Finland, Wales, France, and so forth. Players from the Isle of Man are elligible for any of the Home Nation teams. And conversely, nobody ever has Vatican City as their sole nationality, the team being made up of those who live or work within its walls.
(In real life, a number of people do have Vatican citizenship and only that, but I presume this is because they have renounced their old one).
It's an interesting conundrum. The solution is to ensure that each nation also has optional fields to indicate the available alternate nationalities, forced or otherwise. The Basque and Sami teams are particularly interesting because the lands involved span multiple countries - Sápmi, or Lapland if you prefer, is located in Northern Scandinavia and spans Norway, Sweden, Finland and Russia. Perhaps the best solution for this is to include in the National data the demographics - the rough percentages for each country. More so, it will be essential for the newly generated players in the game to actually come from the correct country - their place of birth needs almost always to be a city in Russia as well as Sápmi, for example.
Of course, it would be considerably easier if I dropped these countries entirely, instead preferring to go with established nations where ambiguity does not exist. But these relatively obscure details are what I feel will make the game stand out.
I want it to be more than just a game.