Oh, don't get me wrong! Having a simple gameplay isn't necessarily a bad thing! It also means that it's quicker for a new player to jump in and grasp the concepts, which is obviously a good thing! Not that I am a game designer, mind me! But my advice is: just make sure that the gameplay is somehow altered after a while, to give the player both a sense of increasing difficulty and an equal rewarding (which is something you already started with those two extra yokai). Keep it up!