If Nintendo isn't going to make more Metroid games then it's up to fans to keep the series going with games like Metroid: Rogue Dawn, an unofficial prequel to the original NES Metroid that's been over two years in the making and which is now finished.
It's a ROM hack which transforms the original into a story set before Samus' "zero" mission, putting you in control of Dawn Aran in a game that looks so good that there are times it appears more SNES than NES.
Video via ShiryuGL
Here's the story pitch:
Rogue Dawn is an unofficial prequel to the original Metroid game released in 1986. 30 years later we bring you the events that unfolded before Samus's famous "Zero" mission on planet Zebes. The original Metroid manual tells us about a deep-space research spaceship that was attacked. Pirates seized a capsule said to contain a newly discovered life form in a state of suspended animation. Rogue Dawn surrounds these events that led up to the theft of the capsule which ultimately placed the Metroid's under the control of Ridley and the Mother Brain. The mission to acquire the capsule is placed in the hands of one of Ridley's best kept secrets. A rogue human agent trained and manipulated by Ridley from a young age. Taken as a child from a Federation colony obliterated by Ridley's faction she was initially just another test subject slated to undergo horrific experiments. Ridley decided to instead manipulate and mould her into a weapon, his first human servant and dark agent. Primed yet untested she is sent out on her "zero" mission to retrieve a newly discovered biological specimen in the possession of a Federation research crew orbiting planet SR388 and prove her worth. Her name is Dawn Aran!
Aside from the visuals, some of the other things Rogue Dawn messes with are missiles (which are more valuable), a new screen-scrolling effect, the ability to jump while morphed and a new save game system.
Note that because this is a ROM hack, and not a standalone title, you can't just download it and play it in an emulator. It needs to be loaded onto a Metroid ROM, but if you know your way around this stuff it's been reported working on hacked NES Classics and Wii virtual consoles as well.