Rain World got a big update recently. For those who don't know it, it's an indie 2D platformer that has unusally intricate AI and procedural animation. It takes a really old-school approach to game design in that it is unforgivingly difficult and gives little guidance to the player, expecting them to figure the majority of the game mechanisms out by themselves (which is sometimes taken too far - a few controlling tricks and the karma system are never properly explained in-game). The update is supposed to make it more accessible by adding an easier mode. There's a lot of emergent complexity created from the interactions of the various beings in the ecosystem, and the AI and random placement of enemies make it particularly tough to solve; you don't really become "fluent" until after 20+ hours of experience. There are no exp gains or power-ups (aside from the karma flower and being able to make the slugcat glow in the dark at one point), and progress is completely dependent on player skill. Here and here's some video reviews.