I think that you need to add like something like cases like in clash royale or some more games and the earning of this cases will be in the end of each raund and the slots of the cases will be limited. the opening of this cases will be for time the time you can decid if someone want to bosst this time he gonna need to pay cristals. from the cases the drop can be anything from cristalas to something like cards and each player gonna need to collectc some cards to upgrade the tank not only cristals i think you can make the game better with the cases