Ah man would I love a Mega Zoroark that dropped Illusion! I'm not sure what ability would fit it, it may be dependent on the theme they choose for the theoretical Mega. (Like if it gains any new typings etc.)
(Or at least give Zorua and Zoroark a hidden ability please)
I really like Zoroark a lot and I want to be able to use Zoroark as Zoroark, not something else, haha!
To fit Zoroark's theme I can see Pickpocket or Magician, but of course that would defeat the purpose of a Mega. Maybe Infiltrator, Prankster or Defiant?