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It's naive to think that designing a mega is as simple as "let's just make this thing that looks like the original form, but cooler, and call it a day". As developers of the game they need to have a reason behind the design that fits the Pokemon and/or universe lore, eg. Salamence's mega focusing on flying ability since that was Bagon's dream, Aerodactyl's mega being its proper prehistoric form or whatever, etc. If they can't come up with something like that and complement it with a good design for a Pokemon, then so be it, it has probably happened with many other mons.