15 Times Fans Left Their Mark on the Official Material

Fans have many ways of engaging with what they love: going to conventions, posting on dedicated message boards, writing fanfiction, hiding in the bushes outside their favorite authors’ home… Most of the time, it’s a one-way, trickle-down thing; but it happens on occasion that some salmon-like fan manages to swim upstream and lay their eggs in their source material (this metaphor went down a weird road, but we’re sticking to it).
It's a beautiful thing - fans get to feel like they're being heard and that their ideas matter and creators get some pretty nifty ideas, as well as cover all-important fan service. If only other areas of life functioned like this - we're looking at you, politics. But honestly, that stream might just be too polluted, but we can dare to dream.
Here we have collected several examples of beautiful fan-hatchlings in canon. And they managed to avoid being eaten by bears along the way.
The Force Awakens R2 was built by hobbyists.

Dr. Who used a fan-made opening.

Sega hired game modders for Sonic The Hedgehog

A fan artist got a job on Detective Pikachu.

A teen viewer created Me-Mow.

Source: Animation Magazine
Fans contribute to Magic: The Gathering

Source: Magic: The Gathering Wiki
The Expanse thanked fans, in-show.

Source: CBR
A five-year-old child designed an Angry Birds level.

Source: GameZebo
A player handed GTA a fix.

Source: Polygon
There's fan art in Doom Eternal.

Sources: GamersPrey (via YouTube), Hypertext
Three teens from Virginia chipped in on Tiny Toons.

Source: Education Week
A fashion-minded fan designed Supergirl's costume in the 70's.

Source: CBR
Fan Written 'Star Trek' Episodes

Sources: Daily Star Trek News, Den of Geek