Primal Legendary Birds
The original Legendary Pokémon trio, the Legendary Pokémon Articuno, Zapdos, and Moltres, were once titans of Pokémon battles, but have faded from popularity since stronger Legendaries with deeper backstories came along.
However, the material outside the core series Pokémon games gives us enough materials to reinvent the Legendary Birds and bring them back with supreme power and a deep backstory.
In the core games, the Legendary Birds are merely Elementals whose physical forms are loosely based on legendary birds from various mythologies and popular real-life bird species.
However, in Super Smash Bros. Meleé, the Legendary Birds are described as the bringers of Spring (Moltres), the summer rainy season (Zapdos), and Autumn and Winter (Articuno), which makes them more than Elemental Birds, it makes them gods like the Legendary Pokémon that edged them off the stage, gods of the changing seasons.
Story proposal: The Legendary Birds-Articuno, Zapdos, and Moltr