My FMP concept idea - 

BACKSTORY -

The story starts with 4 siblings who need to save existence by combining their souls together; The protagonist (one of the siblings) refuses to do this, getting their soul ripped from their body causing them to be soulless. The protagonist then gets cast out to a dark dimension, passing out in the process.

GAME START -

The video game starts with the protagonist waking up after centuries of sleeping; the first thing they say/ think of is the need to find their own soul. They find a random portal, unknown how it came into existence, and travel through it.

They travel through 3 dimensions before coming face to face with the new owner of their soul. Each dimension is ruled by one of the 3 siblings.

 Each of the siblings have their physical body in their own dimension being powered by half of their soul; the other half belongs to the antagonist

The antagonist is the result of the 4 siblings souls being combined.

The protagonist does a boss battle at the end of each dimension with the sibling that owns that dimension, destroying their physical body and stealing their half souls.

The protagonist gains more power after each dimension

END GAME -

The protagonist finally ends up in the dimension face to face with the antagonist. The protagonists soul that the antagonist possesses reacts to the appearing of the protagonist, this allows the player to know what soul they aim to collect. 

After the arrival of the protagonist, each half soul that exists in the antagonists form combines with the half souls the protagonist possess; This makes the antagonist turn into their true form, the final boss battle then begins.

The protagonist then finally possess their soul and their siblings souls causing all of existence to fall.