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 *-*