Platformers, it's a dying art. Banjo-Kazooie, Jak and Daxter,etc. The Ubisoft "Parkour" games while fun, are a poor substitute.
I'd love to do a large-scale, collect-a-thon platformer with a Steampunk feel. Call me boring, but Steampunk is just too awesome. Imagine for instance 10 different Steampunk worlds, in each you collect a number of pressurized valves to open huge Gates to let you into each world. You collect bullets, tesla coils, and various "specialty" ammo like nets and ball-bearings. You collect fuel for your "Hover Boots" which jet hot steam with enough force to propel you forward.
The worlds should probably be spread across classic cities, an antarctic super base, a giant luxury ship liner, and the internal workings of the final boss: A giant mechanical statue of Zeus, the Greek God.
Then there's the level boss fights, which I believe should consist of giant robots. Think the giant spider from Wild Wild West (if you're one of the 3 people who saw that wreck of a movie) only more elaborate, and preferably more creative. A giant steampunk praying mantis, or a snake that travels around on train tracks.
Your villain has to be a historical one, it always works best in these scenarios. I recommend a young Thomas Edison. He's a thief and liar, and perhaps after stealing movie technology from one of the Luminare brothers he moves on to stealing the advanced technology of your main character.
Your main character, I believe, should be a customizable character, a bit like Shepard in ME. ________ Talois, professional steampunk tech-using, Assassin / Thief / Vigilante.
That's my game. I love it. Now... if I only I had any artistic or technical talent.