I do believe that this will come, sooner or later. Still a fairly new game. And from what I see, a successful game at that. So that is why I truly think that InnoGames one day, do not know when, will institute some type of reward system, may or may not include diamonds though.
I have 4 cities in Elvenar, I spent for diamonds my first month of play, mid March. Nothing else since then. InnoGames is not worried about the money part. There are HUNDREDS, THOUSANDS, that play, and have just started, that are spending money here. This is from what I see from the four fellowships I am in and my travels around the world map, mind you. New players and older players, want that beautiful decoration to put in their city. It may give the extra culture that only comes from diamonds or the population you get from using diamonds, but they will get it, no matter the cost. And most have never been in the forum or know how to get to it.
They (InnoGames) collect from diamond sales, put in bank, collect the interest (might want to call it a traders fee). Can you say Ka-.Ching. Look at the cities near the top, notice how much some of them have spent. So yes, a reward and hopefully a few diamonds once or twice a month, is due to all that play in the many Worlds of Elvenar. Hopefully sometime this year. (Hint, hint InnoGames) They may be putting the final touches on some things before implementing this system.