I have a great idea to help achievement hunters, and to deal with unobtainable achievements.
What about every time you complete a retail game you get a gold point and every time you complete an arcade game you get a silver point. A Gold Point can be used to get 1 achievement in any Retail or Arcade game. And a silver point can only be used to get an achievement in any arcade game. But you don't get in game rewards. Like if there is an achievement unlock all costumes you get the achievement but not the costumes. But if there is an achievement for getting an Avatar item you should be able to get the avatar item.