There are a fair amount of fangames and romhacks that exist for Metroid. Although they're not authorized by Nintendo, who cares, they're fun and really good!

The following are some of my favorite ones out there, with the list being occasionally updated for whenever I find a new one that's a lot of fun!

Another Metroid 2 Remake

Metroids.

Numerous incidents in the past have brought to light the highly destructive power of these life forms, spreading fear across all known planetary systems.

Hoping to secure peace in the galaxy, the Galactic Federation orders the extermination of the Metroid species. I was the best candidate for the job.

Once again I find myself standing alone against the Metroid menace. What horrors await me within the depths of their homeworld?

AM2R is a definitive fanmade remake of Metroid II: Return Of Samus, released before Nintendo's very own remake of Metroid II came out. Essentially the best way it can be described is if you gave Metroid II the same treatment that Metroid Zero Mission gave to NESTroid. It adds a lot of new areas, mini-bosses, and upgrades, and much more!

V I T A L I T Y

Samus Aran has been contracted to investigate an installation on Planet Auriga.

V I T A L I T Y is... an interesting one, shorter than most other ones but with a hell of a lot of work put into it. Entertaining with a lot of wonderful design placed around in the world, excellent sprite-work and everything! The ending of it is also very interesting, with a twist that I don't think I've seen much other romhacks do for their games.

Eris

Galactic bounty hunter Samus Aran is sent to investigate planet Eris. The Federation research team's outpost has ceased transmissions. Upon receiving mission details, Samus is reminded of the recurring dreams she'd been having the past weeks.

Dreams of a cold planet and, in the distance, the faint cries of a child.

Could these dreams have something to do with the mysterious disappearance of Eris Stations' research team? The answer lays buried deep within the lethal depths of Eris. Let the mission begin!

Die-hard fans of sci-fi will be pleasantly surprised with how the Metroid canon merges with other beloved sci-fi franchises.

This is an old one I remember enjoying from years ago. Might need to replay it again but I remember enjoying it a fair amount with a lot of interesting locales on a frozen, fucked-up space station.

Metroid: Juicy Mission

This hack was supposed to be something quick in the vein of Samus goes to the fridge, but it quickly became too big and can be considered the canonical sequel to Juicy Mission lite.

So despite the jokey nature of the romhack's title and contents, this is actually a fun little romhack that challenges you a fair bit with finding everything within the world. It may also make you really wish for some Orange Juice by the end.