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Necturna simply does not strike me as a Pokemon that evolved. I put its design and concept in the same class as Skarmory, Absol, Spiritomb, Sigilyph, and so on, badass enough on its own.
Besides, didn't we decide that Sketch would be an event move? If that's the case, then a theoretical pre-evo wouldn't serve any purpose because it wouldn't be able to teach us anything about the game, certainly not what Necturna was designed to teach us.