Toad could be a character in the fanfiction. He could be the messenger or the one who sets up the initial situation that brings Wario and Waluigi together. Captain Toad could also be included. His treasure - hunting skills could be useful in a story where Wario and Waluigi are on a quest for something valuable. And Rosalina could make an appearance, perhaps as a wise figure who gives advice to Wario and Waluigi during their adventures or disputes.