I'm actually a big fan of the Epic War series, especially Two and Four. I think a lot of spin-off games could be made from Elements, "summon creatures, then attack the opponent, kill his creatures. Destroy his castle and use the money you get for upgrades so you can destroy moar castlezz" and tower defense being just two.
For Epic War style Elements, there could be 12 kingdoms of which you choose one, and you have to conquer maybe three cities in each of the 11 remaining kingdoms. Possibly even gain the creatures/spells of the kingdoms you conquer?
To add to the basic ideas:
GRAVITY Graviton Guard
Elite Charger
Massive Dragon
Black Hole (opponent's creature production is delayed for 5 seconds)
Gravity Shield (opponent's large creatures have reduced movement speed)(maybe the mid-range hitters and dragons would be defined as large, basic creatures would be unaffected)
EARTH Antlion
Steel Golem
Basalt Dragon
Granite Skin (increases your castle's maximum health by 1,000)(depends on the scale; not a huge amount, since I assume these spells are run on cooldowns or mana, both of which are reusable?)
Titanium Shield (decreases damage done to your castle by 20%)
LIFE Forest Spectre
Elite Cokcatrice
Jade Dragon
Epinephrine (your creatures' attack speed is doubled for 20 seconds)
Feral Bond (for every creature you have, heal 1 health/second)(may even be underpowered if you have 10 creatures and 10,000 health or something)
I'll add more later, but right now I need to sleep.
All of these numbers just randomly came off the top of my head, so they could use some balancing. I was just putting out the ideas.
I don't think we're pushing to get it developed into a game just yet, but rather discussing the concept and seeing if it sparks interest?