Just a quick suggestion: Add a convenience method in org.bukkit.Server to get an OfflinePlayer object with a UUID string. Currently anyone who wishes to get an offline player if they have the UUID in string form, they have to use this:

Bukkit.getOfflinePlayer(UUID.fromString("<uuid of player>"));

This can be done simply by putting the line above in a method in org.bukkit.Server:

public OfflinePlayer getOfflinePlayerWithUUID(String uuid){
    return this.getOfflinePlayer(UUID.fromString(uuid));
}