A real huge 3D game. If you want to help bitcoin to grow instead of only making profit then you should make a game about BTC and add it to Google Play , It definitely require some funding and time to make a successful game. Ads should suffice to earn income If its really good.
If you don't want to do that then you could build a game but target only bitcoin users and pay them btc while playing which is more simple and require less time and funding then the first idea and the income should be from ads too.