Vaati is a reaccuring enemy (appering in The Minish Cap, Four Swords, and Four Swords Adventures) and is a commonly forgoten character, that does in fact have a sizable fanbase. His character has a range of possibilities for attacks, combos, special attacks, even his form. He would be a far more versatile, and rightfully included character then some of the other villains included (considering he has a wide range of attacks and forms and is the villain of multiple games unlike one-time characters already featured like Ghirahim or Zant). 

To add him to the game would not only please many fans, since he is commonly ranked in the top 10 most desired characters for Hyrule Warriors, but it would also bring attention to the lesser known games he stars in.