With the news that a new Mario Kart on the way to Wii U, I figured I want to share my ideas.
<Gameplay>
I believe the gliders and underwater sections should be brought back from MK7. The same style of drifting is good as well. Some people don't like the hop, so make that removable if they wish. (It's not used for much except tiny shortcuts anyway) Many people have different opinions on 8 or 12 racers, so this should be adjustable in the Options menu. The Wii U GamePad would be used in many ways. You would get a overhead shot of the track like that is in the corner of the screen like most racers, which would be in the corner of the GamePad. I also can't decide whether to have a rear view mirror on the GamePad, in the middle of the screen, or the close-up of the track as seen in MK7 and DS. Customization would return with all of the parts from MK7 and more. Tricks would return as well. We would see the Grand Prix mode, Time Trial, VS for quick games, Mission Mode, and Online Play. I would like to see hand-drawn emblems available too.

<Tracks>
There will be five cups: Mushroom, Flower, Acorn, Star, and Special. All new tracks. The Retro will be Shell, Leaf, Block, Banana, and Lightning. I would prefer more GBA tracks to be included like Ribbon Road and Cheese Land, unlike the whopping 1 GBA track in MK7.

<Characters> All characters from past Mario Karts return, with new characters. For the new characters, I don't have much ideas on me, but lots of people want E. Gadd. I know that's not going to happen. People want Dixie Kong, Cranky Kong, Tiny Kong, and even Kiddy Kong in the game. It's not Donkey Kong Kart people, it's Mario Kart. It's a bit of a stretch to include Funky Kong. Fawful would be a great addition, although not too mainstream. A Luma would be a fun character. Maybe a Noki, or a Pinta. A Hammer Bro would be fun to drive as. Can Goombas drive? I also support the ideas of skins, like changing Mario's color so he looks like Metal Mario, or maybe changing Yoshi's color.

I love Double Dash, and maybe the game can live on by being a mode in the game, like single mode and double mode. Just an idea.
Coins make a return.

<Items>
As for new items, all return, and include a Freezie from Mario Bros.(and GCN's Sherbert Land) that you can throw and temporarily freeze enemies. You could get Spiny Eggs and chuck them forward at foes, spinning them and slowing them down for a few seconds. Make sure the Boo item returns. I thought Star Bits could be a alternative to Fire Balls. I'd LOVE to see the Chain Chomp return or maybe replace the Bullet Bill(that's just opinion.). You could use F.L.U.D.D. to shoot a limited amount of water at foes and slow them down, or maybe make puddles that karts can spin out in. The fake item box returns also. Yoshi's egg returns from Double Dash, and upon breaking, drops three random items like Bananas, Mushrooms, or even Stars. The Rock Mushroom item will turn you into a speeding rock that can flatten other karts, and it lasts a limited time. Swerve into a wall, however, and the rock around you shatters.

That's my take on it. Please feel free to add upon this list or even make your own.

I think the gliding/underwater sections didn't add much to the gameplay. That said, it was a nice, nifty addition. However, I think think Sonic All-Stars Racing Transformed did it better, because you were actually flying and boating and stuff.

They should include some system that gathers star alike power, when the bar is full you can unleash somethin similar to bullet bit power up and star combined. When something hit you, you loose stars so you should gather more stars in order to trigger that power up.

Maybe in the same way some kind of cannon in stylish of double dash.

^^^^ Building off what the prior poster said, maybe something that lets you charge your attack at the expense of coins.

That's a great idea, I never thought of that.

Another nice feature could be something that ign says....taking sonic all stars as a begining....in some moment something would happend.....the bowser armada attacks, a earthquake, something big and the track change in a random way, so not a lot of races would be the same, even different players could have different fates in the same race....mmm maybe a survivol mode.

The weather, in tracks like neo bowser city the rain plays a very important point....so if every track can has a random whether, the skill requiered would be increased.

I'd love to see a track builder.

There is an ongoing discussing of Mario Kart Wii U and what you would like to see here http://www.nintendolife.com/forums/wii-u/mario_kart_wii_u feel free to add your thoughts there.

Great idea. I'd leave out underwater stuff, but keep gliding. Kart customization and coins returning are musts too. Add Diddy Kong, a koopa, Bowser Jr, and leave out just about anything else. Same with items. MK Wii was stupid with so many ridiculous things. 8 / 12 players should be an option, like what was said earlier. Voice chat with friends online would be nice too (not anyone or communities, this shouldn't be CoD with haters, immature kids, etc.).

The rear-view mirror was in SMK on SNes. Having the option of putting it on-screen, even if just for a few seconds, would be interesting, in addition to opting to use the Gamepad for rear-view instead.

How about when a shell is being aimed at you by the guy in the place behind you?

I like those ideas, they should all be put into the next game.

The perfect Mario Kart game would have every character that was ever playable in a Mario Kart game along with the new ones.

Yes but that would be a huge list, almost as big as smash bros.

Thanks for your feedback. I understand it's asking a lot, but they have to pull out all the stops on the next console Mario Kart.

As stated in my post. Great minds think alike!

-No characters other then 16 Toads
-Loads of rubberbanding
-Maps akin to Final Destination/Small Arena (SSB/KIU)
-No modes except for barebones single-player and online
-Only blue shells in item boxes (Thank you TysonOfTime)

You forgot:
-Only Blue Shells in item boxes.

HD graphics, surround sound, 32 tracks, maybe DLC for every stage ever made after the release, DLC characters, good use of the gamepad, new game mechanics, etc.
I don't really like DLC and would rather have all the stuff at once, but they do have deadlines to meet, and also can see something as being valued beyond an initial release be considered for DLC. Plus, it can keep the game fresh if they are continuously adding to a game after it comes out, as long as they don't have it before the release, and just didn't keep it in the game to make more money.

-Large cast of playable characters
-Large selection of courses
-Motorcycles
-Customizable karts & motorcycles
-Land, sea, & air segments
-Double Dash mode
-Online
-Add ons similar to how NSMBU will receive more levels to keep the game feeling interesting and new