To make the races more interesting, and more importantly, diverse, there need to be more variables other than attack, defense and spawn bonus.
My suggestions:
--> Scavengers - 20% attack bonus on surrendered land, -10% spawn bonus
--> Some sort of XP bonus race (lets call them the Eldren), that gets 125% xp, but is as at a disadvantage with the other stats (-5% attack, -5% defense, -10% spawn, or something similar) <-- coin only
--> Similarly, a shard bonus race, but disadvantaged elsewhere (lets call them Shades for the sake of clarity), coin only again
--> A race that allows you to carry more attack boosts, but less defense boosts, and vice versa (shouldn't be coin only, as it would promote people to use boosts). Make it proportional to the map size, the larger the map, the more attack boosts, but fewer defense boosts with one race, and vice versa. Call the increased attack boost ones Fire Sprites, increased defensive boost ones Water Sprites or something.
Im sure i had more suggestions for this, but cant remember them >.> A more diverse selection of races would make race selection more tactical.