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Pretty simple design, but Megas also normally don't change so much they could be seen as a standalone evolution.
Type could very well be Grass/Fairy, the archetype of the royal and wholly good white snake with a crown can be found in european fairy tales. Serperior fits that pretty well. Of course Grass/Ground could also work to symbolize Serperiors further connection to the earth, but the typing is already used by another starter.