Nintendo Sports Mix

Nintendo Sports Mix would be an idea similar to the nature of Nintendo Kart, in the way that it would include not just Mario and his friends (plus the characters from Final Fantasy and Itadaki Street DS), but characters from other games and franchises as well. For example, one of the characters would be Link, another would be Sonic, and others might include Ness, Pikachu, Kirby, Pit, or Captain Falcon.

The beauty of Nintendo Sports Mix comes from its diversity in characters, not only because of the intense excitement that would come from it, but also because of the fact that more characters from more franchises would result in more exciting, interesting stages, such as the Bridge of Eldin.

Gameplay
The gameplay of this would be very similar to the first game of this type, Mario Sports Mix for the Wii. It would have the same settings of 2-on-2 or 3-on-3 team matchups for each sport, and would feature the four sports from the first game, among others. These sports were Dodgeball, Basketball, Volleyball, and Hockey. This game would also incorporate three new sports: Tennis, Soccer, and Golf - three games that were famously covered by the Mario franchise before.

I don't think it would be a great idea to go over the gameplay of each sport individually on this page, so I'll add links to the gameplay descriptions of each existing game the sports are based on.
 * Dodgeball
 * Basketball
 * Volleyball
 * Hockey
 * Tennis
 * Soccer
 * Golf

Characters
As previously stated, the characters would be from far and wide, but because of that, care is in order, because despite how great it would be to have so many beloved figures, you can only have so many in a single game. Pursuant to that, I've come up with a starting roster of characters:

Mario series

 * Mario is the famous protagonist from the Super Mario Series, and he starred in the first Sports Mix which inspired this one. He would have all the same super moves from this game (all of which involve fire).
 * Luigi is Mario's lovable, clumsy brother who would always help him on his missions, but he was also the star of the Luigi's Mansion series. His super moves would involve his same weapon from that series.
 * Toad is a bit of a latecomer to the Mario crew, but his bravery and humor made him an instant member of the team to everyone. His super moves would be the same as in the previous game.
 * Peach is the, you might say, successor to original Princess Toadstool in the original Super Mario games, but everyone has acknowledged her as the current princess. Her super moves would be the same as in the previous game.
 * Bowser is the famed and infamous villain from the Mario series, Bowser has been the fear and bane of many players. His moves would be the same as in the previous game.
 * Yoshi is Mario's trusty steed and great friend. His moves would be the same as in the previous one.
 * Daisy is a relatively new princess in the Mario series, but she has become a huge hit with her dominating attitude and energy. Her moves would be the same as in the previous game.
 * Wario is an extremely popular, extremely conceited, extremely greedy, extremely powerful, and extremely gassy villain from the Mario series. His moves would be the same as in the previous game.
 * Waluigi is a self-centered, but determined businessman who always gets tied up in Mario's adventures, usually alongside Wario. His moves would be the same as in the previous game.

Zelda series

 * Link is the famous protagonist in the Legend of Zelda series, beloved by many fans and feared by evil, known to be the true definition of a hero. His moves would all involve using the Master Sword (and maybe the Hylian Shield), and would involve conjuring many balls and striking them into the other side of the court to fluster opponents, similar to the strategy of the Ninja.
 * Zelda is the magical, beautiful, and sweet princess from the Zelda series, known for her connection to the Triforce. Her moves would involve the Harp of the Goddess, which she would use to send traps in to confuse her opponents before making the final throw.
 * Ganondorf, Ganon for short, is the very embodiment of evil from the Zelda series, constantly striving to become the King of All Evil even through the changing plotlines. His moves would involve making his opponents' immediate area like the Twilight Realm, slowing them down and limiting them, but not unable to stop the final strike.

Kirby series

 * Kirby is the lovable main character from his own series, who constantly adopts different personalities to use against his opponents. His moves involve this principle, as he throws down the same amount of balls as the amount of opponents he has, and throws them, in turn, the same way his current opponents would.
 * King Dedede is the antagonist / awkward ally in the Kirby series, with a somehwat (actually, very) foolish outfit. His moves would involve tossing some of his minions into the other side, and then using his hammer to strike the ball incredibly hard.

Sonic series

 * Sonic is the much-admired, speedy hedgehog from his own Sega franchise, who uses his speed to a great advantage despite his small stature. His moves would involve striking swiftly at the other side of the court while in ball form, and then using that speed to propel the ball toward his goal.

Mother series

 * Ness is the protagonist from the Earthbound series, a normal boy given incredible powers through his paranormal adventures. His moves would involve casting three meteors into the other side of the court, and then hitting the ball where it is least expected.

Kid Icarus series

 * Pit is the energetic, very mythological protagonist from the Kid Icarus series, with great skill in the blades and the bow. His moves would involve shooting three arrows, three times in a shotgun-like pattern to ward off opponents before his strike.

Metroid series

 * Samus is the robotically powered and very mighty protagonist from the Metroid series. Her moves would involve firing an enormous blast into the middle of the court, an unfortunate event for anyone who happens to be in the way.

Pokemon series

 * Pikachu is the little adorable but incredibly powerful main Pokemon from the Pokemon series. His moves would involve casting lightning in a random pattern into the other court, and then striking the ball into the middle of the fray.

F-Zero series

 * Captain Falcon is the secretive, solitary character from the F-Zero series with many enemies, but a thirst for adventure. His moves would involve sending his race car to cause two seconds of havoc in the other side of the court, and then using the falcon punch to cause a disorienting hit.

Mii Characters
It honestly wouldn't be a Nintendo game without the Mii, so it's included, and would have styles of different characters according to its physical characteristics.