As far as I know you can change nationality in football until 21 or you played a senior game. So you can play u-19 games internationally and then change the country. Good example is Ivan Rakitic (FC Barcelona) who played for Switzerland as a junior but then decided to represent Croatia.
This is a really good point that I forgot. That would open things even more.
Croatia (without those who opt for other countries at youth):
Michael Ouzas
Blake Parlett
Luke Gazdic
JJ Piccinich
Mike Aviani
Mario Puskarich
Tomislav Lovric
Adam Deutsch (?...I don't know the situation here)
Borna Rendulic - Nathan Perkovich - JJ Piccinich
Luke Gazdic - Mike Aviani - Mario Puskarich
Tom Zanoski - David Brine - Ivan Jankovic
Dominik Kanaet - Mislav Blagus - Luka Jarcov
Blake Parlett - Igor Jacmenjak
Ivan Puzic - Domen Vedlin
Jan Smolec - Adam Deutsch
Bruno Kegalj - Tomislav Lovcric
Mike Ouzas
Vilim Rosandic