Here in the US I could see the theme being pretty limited in popularity compared to others. I am not so sure about elsewhere though. Seems to me there are plenty of football mad kids out there that will get into the theme and want to continue with it for a while. This isn't the last World Cup by any means, so it may well run in cycles of popularity. I suspect they have a winner with this theme - after all you need two entire teams to play. Same as the Romans - if you can get the kids into the theme then you need lots of them to make decent armies.