I was just thinking a fun aspect would be that there is a random chance(probably small) that a player has one parent from one country and another parent from another country. This would maybe lead to a situation like the bid process on the transfer market except that instead of money, each country would be rated based on prestige(success), potential playing time, etc. Even if it is implemented when the U-21 national teams are implemented, that way once a player is capped on the U-21 team, they will be a part of that country's team when they reach the senior level and they cannot switch. Just my thoughts.

agree totally but more on the side of the player to get there DUAL is by playing in that country for a certain time 3 years for example , instead of having 2 heritage nationality's just have there primary nat. and can get a second by what i stated.and to even play for the other NT's as long as long as it not 2 senior ones kinda like fifa now many will no but can play for a youth and a different senior

Please add U21 NT and U21 cup.

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For example: Dani Alves (Brazil / EU - Spain), Alexis Sánchez (Chile / Non-EU)

But one player should have two nationalities at most.

National team manager can pick a player with two nationalities and have never played for any senior NT, once he was called up by the senior NT of one nation, he is not eligible to play for another one.

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Xavier Chen, he was a RB at KV Mechelen, Belgium.

(http://en.wikipedia.org/wiki/Xavier_Chen)

National team
2001–2002 Belgium U-19
2011–now Chinese Taipei

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I do know that you can change but I was just doing it to work with the bid system that I said. If you bid once(when the player is selected) there would be no changing nationalities, even if he is a U-21). It would be too difficult to let player change nationalities because who would decide that for them? I think for this to work, as soon as a National team is interested(U-21 or Senior)the player goes to "bid"(not for money, but like I mentioned in my original post) at that time the 2 countries can "bid" for the player if they are interested.