Perhaps you could add in different upgrades for plants, Eg, Bee hive plant, Moth spawner, snake holder. Plants in the real world use have alot of insects and reptiles around them, Why not implement them in this? Also, it would be cool if you had to liberate the different continents from the crystals, ending with a boss level/crystal.
The last boss is the exact same from the fourth epic war....maybe change that, and no matter my hero fully upgraded the boss one hits him really making the hero a waste of a unit.
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This is just a great suggestion! I will think about implementing it in sequel! :)