I do not have fan game programming abilities, so I write fan fiction.
Which brings me to the game I would make if I could make a game. I would make a game out of a novel-length fan fiction I wrote (using my own ideas and the ideas of a friend, co-writer/editor).
THE LEGEND OF ZELDA
THE GREAT DESERT
The fanfiction is here, sixteen chapters, around 94,000 words for those inclined to read.
http://www.northcastle.co.uk/guild/fanfic/greatdesert.html
The premise of The Great Desert -
The once beautiful land of Hyrule is now covered by the unforgiving sands. Scorched by the heat of the sun, the green lands are scattered and few. Most people have lost faith in the goddesses and in the legends. In this faithless, dry country, it is time for a young man to bring back the peoples’ hope – with the Gun, the Blade, and the Courage in his heart…
In short, it's a Zelda Western... but, it's not even exactly that. It's still very high fantasy, swords are still foremost and there's even quite a lot of science fiction in the latter chapters. Still, my character design for Link (which I've drawn, I did a cover for the fanfic ) has him in green shirt, dark green cargo-vest and a wide-brimmed dark green cowboy hat. He doesn't use a bow, as he has no need of it when he carries a gun. The gun is his "basic weapon" - the one he starts out with before receiving his various swords. (3 in total in the game, including the Master Sword).
Link begins with an old Western style handgun and his father's sword, which he uses for different purposes, different enemies. Must like in ALTTP, he must gather he three Pendants of Virtue in order to get the Master Sword.
The story takes place in a time "after all the games so far," - hence the increase in Hyrule's level of technology. The fan fiction actually involves things like electricty and television to a mild degree, as well as the "Old West" stuff, but those can be taken out of a game version without affecting the story, I think.
Unlike in the fan fiction, which went along at a fast clip and was character-driven, for the sake of gameplay, the game would have a very vast Hyrulean desert with many explorable areas. In riding from one point to another, there will be little canyons and caves along the way, with stuff to explore - perhaps even Easter eggs to be discovered relating to past games and the history of how Hyrule became a desert. A lot of stuff that seems pointless but is a lot of fun - natural areas to explore, old ruins...
The major problem with a game made from this fanfiction, however, at least officially is that my story deals quite heavily with the past - if you read the fan fiction, you'll see Link meeting, quite blantantly, his "past selves" in a "Hero's Shade" fashion - each one different and specific to past canon games. I don't think Nintendo would ever be so blatant, but with The Great Desert, it is essential to the story. Also, my co-writer and I did some *very* sci fi things with Hyrulean mythology/cosmology and the Goddeesses - While fine for a fan fiction, I don't think it's anything that would fly with Nintendo.