I am developing a turn-based multiplayer app (2 players per match) for Android. As backend I use Google Play Game Services. I have implemented a leaderboard. Now I consider how to design the ranking system - I just realized that Leaderboards in Play Games Services only support increasing existing scores but not decreasing them. Of course that means that I cannot use the Elo system.
Are there good rating systems that only take into account the current scores of both players and do not decrease the score of the user losing the game? The new score of the winning player should be the higher when he was much worse than his opponent and vice versa. I know, that it means there will be inflation :(.