It was recently revealed that Next Level Games, creators of Super Mario Strikers and Mario Strikers Charged (among many others), had a Metroid prototype in the works, but the game was scrapped. As it turns out, that's not the only unused concept they pitched at Nintendo. The talented development team (which plans to work exclusively with Nintendo going forward) was working on a concept dubbed "Super Mario Spikers" which blended volleyball, wrestling, and a game show all together into one game.
Back in 2006, Next Level Games decided to expand their Mario sports expertise to volleyball, and pitched the idea of Mario Volleyball to Nintendo. This idea evolved over time, partially because Next Level Games had previously worked on a (cancelled) WWE game that was to feature sci-fi elements like mech suits. They ended up blending these ideas together, and in 2007 they started working on ideas and concept art for a volleyball/wrestling crossover game with Mario and friends. The project never went into full development (Nintendo apparently decided the game clashed with the company's "code of honor") but a short video of animations (above) and lots of images (below) have surfaced.