After writing 3 pages of ideas and sketches I picked the funnier one that would create more mechanic based on something very simple: Only one eye, leading to "half the eyes, half the screen" mechanic 

https://itch.io/jam/gmtk-2019/rate/460908

I couldn't decide about my only one. So, in my project I used movement in only one direction and only one button for control. Also I tried to use only one color, but actually there is at least two of them: background and foreground. So my game is black and white.

In Ninja On A Budget , you only have a single shuriken to defeat all the enemies!

So you must retrieve it from the corpses after each use... Check it out!

My project was about the last One in a world full of evil Zeroes, and had only one core mechanic, turning on some power switches! These are the little guys

We'd love to have you guys give it a try!

https://itch.io/jam/gmtk-2019/rate/463638

My game is a platformer that uses only one button to control everything.

Mine (https://h-labs.itch.io/only-you) was you being the only thing in the level (aside from the goal). Once I read the theme, I thought "how could you complete a level if you're the only thing in it?" and ran with it.

Through a fun twist of fate, I also ended up with one-button controls.

For my game, "Only one" is the bullet, the minutes and the enemy. 

In the game there is several pnj but only one is the enemy you have to find him in 1 minutes and shoot him with one bullet 

https://itch.io/jam/gmtk-2019/rate/460819

When I heard 'Only one' I immediately went to 'THERE CAN ONLY BE ONE' and the elimination aspect of it, going from having a lot being whittled down until there is only one(or almost only one anyway). Thinking of it like a single player battle royal or a musical chairs like game where you start up controlling a load of characters, until the environment reduces it down to potentially only one as you sacrifice multiple character to solve puzzles to save that one.

From that I swapped musical chairs with zombies that would slowly assimilate your crowd, and while I deiced not to be a stickler of forcing the player to have to reduce there crowd to one person to pass the level, they only needed one person to survive to win. Apart from that, I just tried to keep everything minimalist, colors are mostly all green, all the charters act as one and the like.

My game a quick to play zombie survival puzzle game; https://itch.io/jam/gmtk-2019/rate/461074

For my submission I tried to get as many mechanics into one button as I could.

I made a 2D skateboarding game and I was able to fit jumps, grinds, flip tricks, grinds tricks  and manuals into my only button.

My game's original "Only One" was to create a set of puzzles that required only one key press to solve. And that remained true in the final product.  What did evolve though was how much I wanted to push that central gimmick. I came up with all kinds of weird puzzles by the end that still stay true to the gimmick but also (hopefully) changed the player's perception of it. 

Unfortunately, some of the early puzzles I designed were too obtuse for players who can't read my mind (whoops!). The latter puzzles seemed more intuitive in testing, but my biggest regret of the jam was not revising those introductory levels (the answer to the ladder puzzle is "M" for anyone who wants to check the game out).

https://itch.io/jam/gmtk-2019/rate/461968

My entry's "only one" is the player's only one ability, which is to jump (more than once, though). And, also, the game's playable with only one button.

I intended to have more stages and a recurring boss that you would have to fight in different ways each time (and an one-legged character for extra "one-ness"), but the deadline lazyness struck me hard. Still, I'm glad I managed to submit something in the only one (HA!) Game Jam that I participated.

One hand gameplay, with one input. And.. there is 1 hand model  in game. It is just a simple platformer that allow to play like traditional arcade shooter moving at fast pace, zipping through levels. I learnt a lot of new thing from this gamejam within short period of time.