Meatball is an implied in the Super Mario World cartoon episode Mama Luigi by Mario. When Yoshi eats Mario, he says, "Now I can see how Meatball feels." Yoshi replies, saying that he dosen't like meatballs, and he spits out Mario. Meatball's never mentioned after this.
If Mario knows how Meatball feels when he's eaten, Meatball would be eaten as well, and it's possible that Meatball lives in The Underwhere.
I voted for Wario's Mother, because that sounds like a hilarious character waiting to be introduced and causing all sorts of mischief to out-mischief Wario himself.
Anyway, in that list I have a soft spot for Princess Daisy's father even though he is only mentioned in Mario Party 3, because he sounds like a mysterious character.
Wario also mentioned his mother in those real life talking things they did in the Nintendo store. The parts I remember the most where his mother singing a lullaby to Wario and then scolding him harshly afterward.
"My mother was tough. She didn't want me to turn to like...uh... what I am now. Too bad, mother! Better luck next time!"
Now that Wario's mother is mentioned, I like to imagine that Wario didn't like Shokora as female Wario because it reminds him of his mother, who is implied to be someone Wario might have a bad rapport with.