What we need are awesome cutscenes, great upgrades, and funny dialog.
I've played tower defense games before. Dull. You just create objects to kill moving objects. No real purpose.
Then yesterday I played a tower defense game. Wasn't expecting much. I was blown away. It started immediately with the story, giving background info. Then it launched into a cutscene. It never revealed the full story, only bits so you had to piece it together. Then game the gameplay, it matched the storyline perfectly and you fought creatures. Then came the upgrades which were brilliantly done and thought through. Then came more storyline and hilarious dialog, and more characters, and an in-depth plot.
This game made you have an emotional attachment. That there was purpose more than just shooting moving creatures.
That's what this game needs.