Besides the visuals and art of level design, there are also the meta aspects of level design. That is - how many worlds your game has, how many levels each world has, what's the progression system (complete one level to unlock another one) etc. I recommend checking out SOOMLA LevelUp for this. It's an SDK that provides a rich model of game design objects such as worlds, levels, scores, rewards, gates, missions and schedules. It's available for Unity and Cocos2d-x and both are free and open source.
For proper disclosure I'm one of the founders of SOOMLA.