I would reserve 2 from well established players on the way back up the rankins from injury. (molik, Rubin, Dokic, etc.)
3 for promising Juniors, ( Not based on nationality, results should come into effect)
3 for special use. (Schultz-McCarthy, the NCAA champ is a good one & maybe something like the #1 ranked American Junior or something..
I like how Wimbledon established some new rules limiting certain players from getting WC after WC without doing anything with it... All slams should have some basic rules instead of saying "well you're American so here's a WC" or French or Aussie, etc...