My latest wishlist for a module for Drupal is for an easy way to have users choose, or be assigned, membership in one of a group of groups that would exclude being a member of another group in the same category.

One way to think of this is I'd like to have a set of groups for political party and a set of groups for state. Democrats in Nevada can join the group Democrats and Nevada but can't be in the Republicans group. Ideally a user could change their membership and in so doing would relinquish their membership in the the other group in the same category.