Enter to make a metroidvania game. Yes, that's all...

What is a metroidvania?

From our lovely Wikipedia, we will understand metroidvania by this description:

Metroidvania games feature a large interconnected world map the player can explore, though access to parts of the world is often limited by doors or other obstacles that can only be passed once the player has acquired special items, tools, weapons or abilities within the game. Acquiring such improvements can also aid the player in defeating more difficult enemies and locating shortcuts and secret areas, and often includes retracing one's steps across the map. Through this, Metroidvania games include tighter integration of story and level design, careful design of levels and character controls to encourage exploration and experimentation, and a means for the player to become more invested in their player character. Metroidvania games typically are sidescrolling platformers, but can also include other genre types. 

Simple rules:

• No theme in particular.
• Any  game engine you wish to use(or make your engine from scratch ; -) ).
• You can make  your own art or use Creative Commons or legaly bought assets .