Contemporary game design is built on the blood, sweat, and tears of countless modders. MOBAs, tower defense, realistic squad shooters, walking simulators, survival royales, all started as mods. (For those keeping score: Aeon of Strife for Starcraft 1 or DOTA for Warcraft 3, innumerable tower defense UMS maps for Starcraft 1, Counter-Strike for Half-Life 1, Dear Esther for Half-Life 2, Day-Z for ARMA.)
Add the "prop hunt" to the list. The prop hunt is interior design meets hide and seek, where hiders "hide" in plain sight, disguised as the decorative clutter common in video game worlds, and seekers must guess what is inanimate clutter and what is sentient clutter. Arkane Studios' recent sci-fi shooter Prey (2017) cleverly commercializes this mechanic in a way that modders never could, and while it's nice to know they're paying attention to current trends, I think it's also important to remember the modders that usually get erased from game design histories. In this case, I argue that modders predate AAA design practice by at least 19 years.
CrateDM was a Quake 2 mod released on April 14, 1998, made in an hour by Chris "Shatter" Holden. As far as mods go, it was really simple from a technical perspective: a small map full of crates, and a PPM (plugin player model) that let players appear as crates, and no custom code or anything... which, I think, is brilliant. We often argue video games are made of code, and here was a new game genre created without any code at all!
Here is Holden's process and notes from the readme.txt: