How about a complete departure from the typical TD game?

There is one corner of the board where there is a cave. There is another corner of the board where there is a castle. Various creatures leave the cave (randomly, but with a set number of creatures until teh next level of creature comes out), and proceed half hazardly toward the castle.

Now, the TD part:

The only “Tower” available in the beginning of the game is the “Villager Hut”. Peasants leave the hut with pitchforks and attempt to fight the creatures. Each hut has a limited number peasants that live in it. Peasants have limited hitpoints, and do very little damage. If a peasant is killed, another peasant will grab him and carry him back to the hut to be healed.

We don’t know if the peasants are looting the creatures as they kill him, what do we care? At set times during the game you collect taxes from everyone that is still alive in your kingdom. This money can be used to upgrade your “Towers”

Upgrades include (and this is just brainstorming), Warrior Barracks, Mage Tower, Missionary, and Peasant Hut II. Peasant hut II can upgrade into upgrader “Towers” (Merchant, Blacksmith, Library, Church)

Once a tower is upgraded, the peasants in the tower become heroes, and gain experience and levels as they kill creatures.

I’ll post more if I think of it, but this is some groundwork at least.