Sinnoh is going to be the longest region because it has the most legendaries
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He could try that but it depends on if any other users that try to help him with the game does not make it worse along the way, so he probably only go for whichever ones he trust the most and have the most programming stuffs for making that game, it is up to him whenever he wants to try and see if anyone else wants to help him with this or not.