I feel that this game could really use a referral system. It will increase the player base, and also, provide a possiblity for a paying base for some players. The main argument from this is that the system could be abused. Well, I feel a great solution to this is to put in the registeration field. "Who referred you?" where an iczer's name can be placed.
Now the idea behind referrals is that a player will receive rewards for referring players to the game. My suggestion is that those rewards will only occur if the new player begins to pay. I would suggest some type of reward for maybe every $10 the referred player spends. IE. extra gran, point cards, maybe a free level up card choice.
The idea is that if a person places a post in a forum or something and has maybe 10 people sign up for the game. If only one person becomes a paying player, then they will only get rewards for that one person. The other nine will just add to the player base, which is still helpful. Also, this will give both free and paying players incentive to refer new people to the game. It will give advertisment by word of mouth and give the possibility of a lot of paying players to join.
My thoughts on what should be the rewards would be either 50 gran per $10 spent, or 10 point cards per $10 dollar spent(could also just be 1 point card per dollar), or a "level up" selection per $10 dollars (rarity 3-5*). This would prevent abuse since the reward only happens if the new player begins to pay, meaning that creating a lot of false accounts would be worthless.