In game the girls talk about them being commanded by,"The First Star". Taking into account that Soraka is ancient, and is a being of the stars who acts, or rather acted as their medium on Runeterra. The stars speak to her, so it would make sense that she was born from the will of "The First Star". Upon looking more closely at the composition of the Star Guardians, each of the girls chosen embody a personality trait an emotion, but from what I can see there are two major traits still missing.
Lux embodies a purity, a drive to better oneself and obviously light/good.
Janna embodies level headed thought and freedom.
Lulu embodies and wonder and innocence.
Poppy embodies bravery, duty and honor.
Jinx embodies Rebelliousness and chaos.
Now Soraka could be the original guardian who, at the behest of the first star, bestowed the others with their status of guardian. Soraka would embody love and compassion which is missing from the group as a singular guardian. The other trait missing is the flip side which Dianna does make a good fit for, rage, vengeance and a longing for purpose. Dianna's story of fighting in search of purpose and for revenge for the oppressed Faith of the Lunari gives her a good tie in to the stars.