I have a few suggestions:
1. Each country could send someone (for arguments sake, let's say me) the hottest brunette female they can muster. I will then rate the nations based on, I don't know, let's say the "performance" of each girl... But before you put that potato sack over the random slag from Bigg Market and bundle her into the boot, I like classy ladies.
2. Base the rankings on a "friendly league" (where nations are divided into 1-9, 10-19, 20-29 etc etc and play mostly each other in international breaks) and then on tournament performance. That way it would pretty quickly establish the better teams, and still leave space for playing other teams for the money/diplomacy etc.
3. I like the first one best.