Started 1 hour late - but got my idea now! Will start to visualize it with some doodles, and see what i can come up with.

For anyone still brainstorming here are some ideas i had:

(Un)limited Backpack You have to fit items in a backpack, but there are also backpacks you can fit inside that provide more space than they occupy and able to be stacked inside each other. Therefore you have limited space, but its unlimited! Combine with simple battle and stats system where you loot weapons and armor that you have to store to increase your stats to survive fight which drop even more loot and so on.

Chicken Breeder - Limited Edition You have a chicken farm that can fit 3 chicken and a cage on a pole that can hold another chicken. You have to cross breed chicken in that limited space to find all the combinations and finish with the golden chicken!

Edge of the Universe You are a spaceship equipped with an enlarge-ray. You are able to absorb power from suns to fill your battery and use it to increase your size. There are black holes, space nations and space creatures that can hurt you, and the bigger you get the more suns you need to start the ray again. In the end, you are so big that you barely fit into space and can actually see the edge of the universe!

Survive under the Dome You live under a protective dome, every night monsters attack the dome but the townspeople protect the town. Your job is to fix the dome during the day - but you cant leave the dome to do so - so the more you fix it the fewer your space gets. You get material from the scout squad, but its random size and gets in one by one. Try to survive as long as possible before you have no more space to repair your dome!

Feel free to use any of those ideas as is or tweaked, and leave me a comment so i can test it when its finished!