Well, of course it won't be called MK8, but yeah, I have some ideas:
Characters wise, I would love a mix between the MKWii and MK7 roster, with new ones, of course:
(Unlockable ones are marked with an astersik)
Light Characters:
-Toad
-Koopa Troopa
-Baby Mario
-Shy Guy*
-Dry Bones*
-Toadette*
-Baby Luigi*
Mid-Light Characters:
-Peach
-Daisy
-E. Gadd*
-Diddy Kong*
-Monty Mole*
-Lakitu*
Middle Characters:
-Mario
-Luigi
-Yoshi
-Birdo*
-Kamek*
-Bowser Jr.*
Mid-Heavy Characters:
-Waluigi
-Donkey Kong
-Rosalina*
-Wiggler*
-King Boo*
-Dry Bowser*
Heavy Characters:
-Bowser
-Wario
-Petey Piranha*
-Metal Mario*
-Whomp* (as the WTF character xD)
-Gooper Blooper
There would be 12 characters per race, as in MKWii.
Coins would also reappear, giving you speed and unlocking new things, but in a different way as it was in MK7: On the game's main menu, is the "Shop" option. Once you enter, you can use the coins you've collected to buy new characters and kart parts. However, first you have to unlock them, by beating cups, time trial record, mision mode's levels and getting star ranks. So, first, you have to unlock the content, and then you have to buy it with your money
IMO, that would add a lot of replay value, as even if you beat a difficult quest, you still need the money.
Also, they will act like Wumpa Fruit in the Crash Bandicoot Racing series: once you collect the max of ten, the items you collect get upgraded. I'll talk about that later.
Seeing how the WiiU will have an amazing capacity, Nintendo should add two more cups: the Shine Cup for the Nitro Grand Prix and the Coin Cup for the Retro Grand Prix (they come before the Special and Lighting Cups respectively). This way, the game would feature 20 new track and 20 old ones. I'm not gonna try to imagine new tracks right now, but these are the ones I want to see returning:
Shell Cup
-DS Figure-8 Circuit
-GCN Dry Dry Desert
-SNES Donut Plains 2
-Wii Toad's Factory
Banana Cup
-3DS Koopa City
-GBA Boo Lake
-Wii Moo Moo Meadows
-DS Mario Circuit
Leaf Cup
-N64 Wario Stadium
-DS Tick-Tock Clock
-GBA Ribbon Road
-SNES Vainilla Lake 2
Coin Cup
-SNES Bowser Castle 2
-3DS Piranha Plant Pipeway
-GBA Cheep-Cheep Island
-N64 Yoshi Valley
Lighting Cup
-3DS Rosalina's Ice World
-Wii Gumble Volcano
-GCN Bowser Castle
-N64 Rainbow Road
I picked 3DS tracks without main glider section because I doubt Nintendo would reutilize the glider in future games. The end of Piranha Plant Pipeway could just be a cannon. However, underwater sections reappears, and even are in the new courses.
Now, talking about karts, I mentioned earlier kart parts. I'd love that you can still customize your kart, but with more parts, something like in Modnation Racers. There, you could select not only the frame and wheels, but also the seat, motor, add some accessories (but in a fixed place of the frame), and the emblem, just like in MKDS. Also, each part (mainly the body frame and wheels) could change it's color scheme to match the character, something like MKWii.
About kart frames from past MK games, I'd love to see the Pipe Frame (Classic Kart) again, along with the Bullet Blaster (MKDD), the Shooting Star (MKDS), the Flame Flyer (MKWii) and the Sode Jet (MK7).
Time for items. As I said earlier, getting 10 coins upgrades the one you've got. Here the list, with an explanation of said upgrade:
-Green Shell -> Spiny Green Shell: Can break objects he found in his way, like bananas and other shells
-Triple Green Shells -> Triple Spiny Green Shells
-Red Shell -> Spiny Red Shell: Can break objects he found in his way, like bananas and other shells
-Triple Red Shells -> Triple Spiny Red Shells
-Banana -> Giant Banana (From MKDD, but not as big): Can defend against Spiny Green and Red Shells
-Triple Bananas -> Triple Grian Bananas
-Spiny Blue Shell -> Winged Spiny Blue Shell: It goes for every character in fron of the user before hitting the one in first
-Super Mushroom -> 1-Up Mushroom: The boost last twice a normal one
-Triple Super Mushrooms -> Triple 1-Up Mushrooms
-Golden Mushroom -> Electric Golden Mushroom: Anyone you hit while using it's boosts will get shrinked
-Super Star -> Rainbow Star: It's effects last twice a normal one
-Lighting -> Thunder Cloud: The cloud stays above you, giving a small speed boost while the lightning effects lasts
-Blooper -> Blooper Nany: The Baby Blooper will slow down the affected characters
-Bob-Omb -> Big Bob-Omb: It has twice the explosion radio of a normal one
-Bullet Bill -> Banzai Bill: Bigger, so it has more chances of hitting some one; also a little faster
-POW Block -> Red POW Block: Creates two earthquakes
-Fire Flower -> Ice Flower: If someone gets hitted by an ice ball, they'll frooze for a second
-Super Leaf -> Stone Leaf: Makes you heavier and automatically protects you from Green and Red Shells
-Boo -> Dark Boo: Steals one item and three coins from a random character
I couldn't think of any new item. But with the upgrade system, maybe it doesn't matters
And from items to Battle Mode. It would be nice if the battle stages gotten organized in cups, like the races. They could be just two, one with four new stages, and one with four old ones.this way, you could unlock things by beating these cups in both Baloon Battle and Coin Battle Mode. These are the four stages I'd like to see returning:
-SNES Battle Course 1
-GCN Luigi's Mansion
-Wii Chain Chomp Roulette
-3DS Sherbet Rink
Mission mode will always be a welcome addition.
And yep, I can't think of nothing more right now. Sorry for the boring stuff
