If people play on Co-Op, does it just go on forever like Terraria? This is coming from your statement "At the beginning you and the other players start in the surface of the world and go deeper and deeper, improving your armor, learning new abilities and fighting bosses until just one player or team survives," which I'm assuming is when it's team based. Also, when you're animating the sprites for this, please make the sprite animations fluid and not stiff like your inspirations. As much as I like Terraria and Starbound, the character animations are lacking in the body movement department.