As we all know, ever since the legendaries became a central point of the game's story they have included a third legendary to the main duo to balance things out
>Hoenn legendaries represent the ecosystems as land and water
Soaring dragon that puts an end to aggresive expansion and lives in the sky
>Sinnoh legendaries represent the dimensions of time and space
Antimatter dragon that distorts reality and was sealed in a different dimension
>Unova legendaries represent humans' view of truth and ideals
Inert dragon devoid of energy that waits to be filled by its former components
>Kalos legendaries represent life, as in its beginning and its end
Underground dragon that conceals itself until alien beings menace the planet
>Alola legendaries represent how light is emited and reflected
Crystalic dragon that refrects light and devours or generates worlds' light
>Galar legendaries represent hand-gear for offensive and defensive use
Hand-shaped dragon that causes havok with its mere influence and prowess

>PokeSpain legendaries represent the eras of the distant past and the future
What could the third legendary (dragon) be this time? Eternatus was already pushing it too much, it barely fits with Zacian and Zamazenta.