Take a trick from league of legends book. Sell skins. Me and my wife were just talking about how we'd never really buy gems but we couldn't throw money at our screen hard enough over skins.
Skins are purely graphical changes. They are just neat and extra and have no game play impact.
For example, you could do a "cat" skin. Make all the troops look like cats, Make the TH a cat tree, Archer towers do lazer pointers, etc etc.
My wife said she'd pay $15 for this easy, pretty sure every other girl is in the same boat.






