Frozen 2 introduced elemental spirits into its universe and represented them in some very particular ways (A salamander, a water horse, a gust of wind and multiple earth giants), and then centered much of its story around them, with Elsa ultimately becoming the Fifth spirit.

Was the inclusion of elemental spirits a logical addition to the Frozen lore?
What do you think of their implementation, both story and design-wise?
And what about Elsa, a human with ice powers, somehow being the fifth spirit when the other ones represent the 4 elements?