Legendaries are legends,myths, and real. Too much of one thing is bad, however. There are many ideas to base the origin of a legendary, such as Celebi's purpose is to be protector of forest, meanwhile Groudon creates land and earth, and Kyogre creates water(or makes) and oceans spread. Some have ridiculous roles.
Name a few roles or based origins some legendaries might take on the future:
I believe, that a legendary out there might end up a role based on HumanKind, Animalia, Insectoids, etc. They made them based on weather, elements, they can make them based on this. Opinions only.