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The whole reason Sugi got artist's block on Flygon was because he was being asked to design a 4th stage of evolution for designs he'd already brought to conclusion, in his mind. Megas deserve to stay dead, as they were tacked-on atrocities that could never replicate the conditions that resulted in their base forms' creation. Hell, Meltan and Melmetal did a better job of being an addition to an older generation as it felt like it could have been created back in Gen 1.