Golf, Tennis, Baseball, Soccer, Basketball, Volleyball, Hockey, Dodgeball. These are all the sports that have been made into Mario games. People often joke about Mario playing every sport under the sun, but if you had to pick, what sport would YOU like to see Mario play next? Feel free to elaborate on how such a game would work given the Mario twist. Here's some of mine:
Mario Bowling
Surprised this one hasn't been made already. I've always wanted to see a Mario Bowling game of some sort. They can introduce wacky lanes, wacky bowling balls and wacky power-ups to the mix. I recall there being other bowling games with similar premises such as Milo's Astro Lanes for the N64 and that one Animaniacs bowling game fro the PS1, so Mario doesn't seem like a bad fit.
Mario Gridiron
Another surprising one. They could probably get a western developer to make it, much like how Next Level made Mario Strikers. I'd imagine it being one of those less complicated arcadey football games like Play Action Football or Tecmo Super Bowl. Would also give them a good reason to bring back Chargin' Chuck.
Mario Handball
Handball is already kind of similar to tennis, so this shouldn't be hard to picture. Just have power-ups on the field and special targets on the wall. Plus, a bunch of Mario characters wear gloves already so they'd be used to it. :P
Mario Fishing
You might say fishing is too "realistic" or "boring" a sport for Mario, but one could easily say the same thing about golf and Mario has had more golf games than any other sport. You'd think they'd have made this by now considering how prevalent fishing mini-games are in Zelda as well as how much Japan seems to love fishing sims. There could be a mode where you catch real-world fish and another mode where you catch Mario baddies like Cheep Cheeps and Bloopers.
Bowser Big Battle Wrestling
OK, so Mario's not in this one. I figure Bowser would lend himself well to some sort of wrestling game. Preferably a wrestling game like SNK's King of the Monsters where all the competitors are massive and wreaking havoc on a defenseless landscape. Other playable characters could include Donkey Kong, Wart, Tatanga (in a Pagosu suit), Smithy, Midbus, and maybe Ganon as a guest character.