Just had a read through and i personally think it should be a first come, first serve basis when the new game thread is opened.
The top 5 can get to choose if they want to keep their country but maybe put a cap on this. Let's say poster A has Australia and their entry finishes 4th. The new thread opens but they don't post for a few days but i want to take Australia i don't think it'd be fair for me to have to wait until poster A confirms days later they want a different country (or don't post at all). Then someone else takes Australia even though i could've confirmed that country on the opening day or at least on the second day if poster A wasn't around for 3 or 4 days to claim it.
Just my opinion.