Do you think there should have been a Fairy-type that got Illusion as an ability? Not necessarily an existing one, just as a concept in general.
Hrm. It makes sense, don’t get me wrong – fairies go well with illusion and deception – but I think the Illusion ability is something that it’s better to keep as Zoroark’s ‘thing,’ both for Zoroark’s sake (because, to be honest, it would probably not be difficult to come up with a Pokémon who gets more out of Illusion than Zoroark does) and to avoid the chaos that would be brought about by having both of them on the same team. That’s not to say we can’t try to think of something that draws on a similar idea of deception, though… fairies from different cultures and different stories can have control over a wide variety of elemental forces, so… maybe a Fairy-type who can have several different secondary types, which the opponent can’t immediately identify. It can be (say) Fairy/Fire, Fairy/Electric, Fairy/Water or Fairy/Grass, chosen at birth, and its appearance is the same for all four (maybe including elements from all four types), which means that the opponent has to fire off one or two “test” attacks to figure out what its weaknesses and resistances are. Undecided on whether to have moves that are unique to each form or just give all four a wide range of attacks. Could go either way there.