Hey, I came to the same conclusion.
In the first time, I though about adding new kinds of enemies such as shooting enemies or bomb enemies, but these features need more new features in the player side like landmines or melee units, in order to give the player a chance to win.
The issue is not in the production of these features: I can add them in a few minutes. However I didn't find a way to show the differences between units, walls and enemies types.
Nevertheless, once I find something, these features will definitely be a part of the game.
Thanks for your comment and have fun, bye