Hello there,
Congratulation about your cooperation with your client.
So from the project description that you explain, I can see that you are interesting on creating endless casual game. There are some plenty of casual games out there. For the example you provide, Flappy Bird is one the luckiest app that reach the top of the user attention. Yes, it is a luck, but followed by several factor that provided by flappy bird. Simple gameplay, simple yet unique graphic, simple interaction, simple game iteration, etc. This flappy bird game create a new trend of casual game.
But today, there is a potential type of game that adopted from simple casual game like flappy bird, but it is injected by several concept that could increase the interest of the game. It is called Hyper Casual Game (Maybe you already heard of it). Hyper Casual game have a very simple interaction, it could be tapping only, swiping only, etc, yes, usually it's only consist one interaction. Simple gameplay, the gameplay pattern are likely to be repetition. But one think that makes it interest point, is the graphic. The graphic are quite simple but catchy, juicy, and satisfied to watch.
Can you tell me more about the specification of the game is? Remember, the key is good execution and simple to play... Love to know more if you would like to chat about it.
Best Regards,
Haidar Abhirama