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Zoroark's problem isn't Team Preview, it's simply that it sucks statistically. Illusion in itself is actually still a powerful tool even with Preview (especially now that boots exist), it did in fact earn Zoroark a ban from RU in the previous two generations.
But perhaps the best example of Illusion's potential is not Zoroark in lower tiers but this guy. Smogon maintains a side metagame with their own original creations (many of which tend to be significantly more powerful than canon Pokémon). This guy, Aurumoth, is a Bug/Psychic pseudo with very wide coverage and a lot of boosting moves. It used to have Illusion as one of its abilities (alongside No Guard and Weak Armor), they eventually replaced it in gen 7 with Light Metal because it was just that blatantly broken, even with the higher average power level.
