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With XY they successfully built up engagement starting with Mewtwo Y. Nobody knew what this was or what it meant. But Millennials remember "Mewthree", and old playground rumors start to re-emerge. Kitakaze the artist for Mewtwo Y was one of those very kids who loved Mewtwo in the 90s, and was inspired by the movie. The "Ultimate" version of Mewtwo, small and cute making it more similar to its ancestor.
Once we got the news of other Pokemon having special forms like Lucario, Blaziken, Gardevoir...Other fans start speculating, discussing stats, making their wishlists, fanart, it was amazing. Giving every Pokemon the exact same power up, it's just not the same. It doesn't surprise me that the other gimmicks were so quickly discarded.