I'd call it Cappy Cup if you ask me. :P

ok

How about give all of them characters as well, to make the DLC more appealing:

Bowser Shell Cup: Funky Kong

Rainbow Cup: I guess you could give Toad 9 colour palettes, similar to Yoshi & Shy Guy

Cappy Cup: Pauline

Ooh and you could also add items!

Bowser Shell Cup: the Bowser Shell from earlier games

Rainbow Cup: Fake Item Box, also from eariler games

Cappy Cup: a Cappy item, works like the boomerang but you get 2 hits and when you hit someone you steal their item/s

Toad Colors:

1. Blue Toad
2. Yellow Toad
3. Green Toad
4. Pink Toad
5. Orange Toad
6. Purple Toad
7. Red Toad
8. Black Toad
9. Captain Toad

oh yeah you can't have a white toad

So I had to use a Red Toad, same as normal toad but with a red shirt.

A return of Moonview highway would be nice

Ok, I can make a cup with Moonview highway if you'd like:

Inkling Cup:

• Inkling City
• Moonview Highway but it's splatoon themed, renamed Splatoon Highway
• Koopa Cape but instead of water it is ink
• Coconut Mall but with an Inkling theme, renamed Ink Mall

The character that come with the DLC would be the squid sister (in a skin pack, similar to Link)

The item would be a splatter, it would create a small trail of ink that functions similar to the banana peel

ok, I just don't like how Link, Villager, Isabelle, and the Inklings appear because even though they're cool, we can't have Mario Kart become Nintendo Kart (Nathaniel Bandy suggests replacing Mario Kart with Nintendo Kart, but I think that what they can do is make Nintendo Kart a spinoff of Mario Kart)