I've had several ideas, some way beyond my skill, some I have tinkered with on Game Maker.
My absolute dream game to make would be a Bethesda-style western RPG set in a world destroyed by Lovecraftian style inter-dimensional creatures. A few years back I wrote a full length choose-your-own-adventure book where I came up with the skeleton for the story and through-out the years I've been jotting down ideas for quest, main story and setting although sadly I have no skill with 3D modeling. Basic plot involves a large countryside area surrounded by mountains that commonly is attacked by lesser interdimentional horrors, with legends of greater ancient gods who merely looking at causes instant madness. Story begins when one of these greater ancient gods appears in your village, instantly destroying the mind of everyone who sees it, but your character sees it and somehow survives, giving them power over madness. The countryside area contains scattered villages and towns and ruins. Later, it is reveal that outside the mountain range in all four directions are the ruins of huge cities, completely overrun with monsters and worshipers of 4 greater ancient ones:
Icha'Wachi, an entire dimension made up of it's horrid internal organs
The Dreamer, a giant tentacled eye-ball creature keeper of knowledge in it's own real, The City of Ice
Fantos, a mindless apex predator in a realm of horrible creatures
and one more I haven't named yet, who cannot die but outside of it's own realm it's body decays at a massive rate, causing it to constantly and haphazardly sticking it's body back together outside it's realm.
It is discovered near the climax of the main story the reason the outer cities are so much more infested then the countryside is it resides on the body of a giant ancient god who might see people living on it's body like parasites when it awakens. After it is dealt with at the end, it is revealed the countryside and cities where ripped from a larger world being within the giant ancient god's realm, depositing it back into a world of endless corrupted ruins of human civilization (Think New York City covering all land in every direction).
(https://dl.dropboxusercontent.com/s/mh9cfhrc0mtzg3i/TabblewoopIICover.png?dl=0)
This is the cover of my Choose Your Own Adventure. The middle row contains Icha'Wachi, The Dreamer and Fantos. The other two major ancient gods I came up with after this for the game.
I've written like 100 quests for this, but they'll sadly likely remain on paper only.
Another one I actually can do with some work is my idea for a light-hearted cowboy themed Metroidvania. It'll have all kinds of ridiculous animals and non-sentient things as western themed enemies in a largely comedic adventure. I can do everything required to make this game, though i still have trouble getting 2D physics to work perfectly... I can never seem to get it 100% glitch-free. I also am somewhat skeptical of my own artistic ability... I could make all the sprites look passable, but nothing like the 16-bit art I would really want.