Nintendo has tons of characters, and of course not all of them can have the main spot in the games, but it's likely that you've been surprised from a certain side character, and thought "Wow, I'd love to see this character in it's own game to learn more about it/ because they could do cool things with him/her" .

So who is the character that you think deserves it's own game ? 

For me it's Ashley from the Wario games, I love her for the personality they gave her and I'm shure they could make an incredibly creative game with a witch. They could even make a crossover with Luigi's Mansion *-*

Palutena

EDIT: Or Zelda

Nikki of Swapnote fame.

Waluigi can have his own casino - that should make for a nice spin-off.

I agree with Ashley, too. She's cute.

Oh, and SEGATA SANSHIRO.
I don't care if he's not a Nintendo character, or if he'd be stupidly overpowered, I want him in a game.
SEGA SATA-N, SHIRO!

Sheik/Zelda. Make it a magic centric game where Sheik is the heroine, using her badass magic to rek da scrubz.

Shy Guy should get a series of awkward dating sim games.

King Dedede
Kamek
Sheik
Falcon
Krystal
Midna
Birdo
Waluigi
...

its time to let this character flourish

best idea ever!

The tennis ball in Wii Sports.

Bowser, deserves his own platform series :)