I say:
-Bring the 12 racers back.
-Bring the Double Dash mode back, but unlockable. This way you can race both individually and in teams.
-36 characters, this being the roster:
1) Baby Mario
2) Baby Luigi
3) Baby Peach
4) Baby Daisy
5) Toad
6) Toadette
7) Koopa Troopa
8 ) Paratroopa
9) Dry Bones
10) Lakitu
11) Toadsworth
12) E. Gadd
13) Mario
14) Luigi
15) Peach
16) Daisy
17) Yoshi
18 ) Birdo
19) Diddy Kong
20) Dixie Kong
21) Bowser Jr.
22) Hammer Bro.
23) Shy Guy
24) Magikoopa
25) Wario
26) Waluigi
27) Donkey Kong
28 ) Funky Kong
29) Rosalina
30) Lubba
31) King Boo
32) Petey Piranha
I would have added Dry Bowser and Queen Bee, but including them would break the "12 characters per size category" rule, so...
-Make the cups have 5 races instead of 4 like it was in Super Mario Kart.
-Add two brand new cups: the Shine Cup and the Coin Cup.
-No more retro tracks, this meaning that all the cups would be made of brand new courses, meaning a total of 50 brand new circuits.
-Bring back ALL the items from past installments, including the Thunder Cloud, but this time around, there should be a way to get rid of it without having to collide with someone else.
-Mix all the features from all games, including diving, the motorcycles, gliding, etc. (It would be weird to see how a motorcycle being ridden by two people looks with a glider on... Hahahahaha)