I'd love for the Mario sports games to tackle football. That is...

Nintendo Football

There's tons of Nintendo-universe characters out there, so we could have about 8-10 teams. Even one per universe, perhaps.

Mario Team (Mario QB, Luigi WR, Bowser, Toad WR, Wario)
Zelda Team (Link QB, Tingle (haha) Gorons, Zoras, and Hylians mix)
Fire Emblem Team (Roy QB, Marth, Eliwood, Lyndis, Hector, Ike, etc)
Pokemon Team (Mewtwo QB, Pikachu WR, Snorlax, Rapidash, etc)
Star Fox Team (Fox QB, Falco, Slippy, Krystal, Peppy, Bill, Katt, Star Wolf)

and I can't think of more right now, but there's more.

It could play more like NFL Blitz or something, depending on the amount of characters eventually collected.

Dodgeball. Mario Dodgeball.

A glorified return to the Mana roots, the ultimate sequel to Secret of Mana!

Or are we talking original designs? Here's one:
For the Wii

You play as a teenager who has great hand-eye skills. Using the Wiimote and nunchuck, you work a job at a circus as a juggler. Things get harder and harder, first going from 3 to 5 balls, then juggling pins, swords, and fire sticks! To make things harder, other things can happen around him, like an act goes terribly wrong, or the crowd throws tomatoes at you.

It would also be like a grand theft auto world, where you could use your skills to help civilians. Imagine going to the grocery store. A lady drops a package of eggs. Time slows down, and you run, make a jump, and grab the eggs for her! Yeah!

Oh, I know a perfect one!

Okay, it's an RPG about time travelling. You start off just like any normal person who gets sucked into the past to save the future. You'd travel across many periods of time while running into plenty of people in each. There would be a deep storyline with plot twists and multiple endings. It would also have great replay value and...

Oh wait...

Slayers' Reign. But that's why I'm making it.

I've said this in other topics, but I want a Mario game that:
1) Has a hefty amount of fun power-ups, like in Mario 3. A flying power-up is required, as well as a neat outfit change.
2) Has Yoshi as a power-up, like in Mario World and Mario Sunshine. Except I don't want Yoshi to be allergic to water or be on a timer. It just doesn't seem right if Yoshi isn't carrying Mario or whoever on its back.
3) I want to go from Point A to Point B, not go into a level 8 or so times to gather stars sitting in random places. While they're at it, they can include the occasional well-hidden secret exit, like they did with the Mario games way back when.
4) Bring back the Koopalings.
5) Mini-games and sub-levels like in Mario 3. You know, card battles, item houses, mini-bosses like the Hammer Bros. battles, and so forth. And perhaps something like the Switch Palaces in Mario World.