Sona's Parents Theory
After reading her lore again I thought one of two things.Her parents may not be relevant to the lore or they were but just there is no way to introduce then.
It's possible Sona's Parents are quite "something" as evidenced by them leaving Sona the Etwahl and Sona having a special connection with it.Or they may have been nobodies who just couldn't raise Sona,but if they were nobodies why did they have an Etwahl to leave with Sona?
What we know is Sona is born in Ionian but she resides in Demacia.We know in Ionia magic is respected and loved and not feared.Which means if Sona displayed "magic" like tenancies she wouldn't have been given up so easily.Sona's parents may have given her up maybe because they couldn't keep her or she was a love child of someone really important in Ionia who was targets of Noxian assassins.Or maybe they just didn't want her.But again it is entirely possible they just didn't want Sona.
Another theory I have is,the fact Sona has a connection with her Etwahl and that it killed Sona's adoptive mother Lestara Buvelle out of jealousy may mean,it is a sentient object or bear with me may or may not have the soul of Sona's Parent/s' soul residing within or it is her guardian.All we know is the Etwahl is something powerful and the only thing we can gather is Lestara knew about this Etwahl what it does,or is capable of.
Sona's whole theme involves her being mute,her music and her Etwahl being her "voice".If there was a recurring theme of hers involving her true identity and parents it may be possible Sona was turned mute by parents because something was wrong with her or because it was to keep her safe.Who knows we can only speculate but if I were to give Sona a backstory or something to give her a connection to her parents.
I would say she is the daughter of powerful or high ranked people in Ionia who are targeted by Noxus.They gave her away to keep her safe and the Etwahl is her guardian or protector of sorts.She is mute probably because something they did to her or something that happened to her.If I were to give her parentage to champ ions in League I would say Lee-Sin is late 30s to 40s,we know Sona is in her late teens to early 20s.And Lee-Sin is the only Ionian I can think of who might have a child or love child.I get that he was a monk but they aren't going to go into deep detail of everything a champion did in their past.Its possible he had a love who was a musician hence why Sona has an Etwahl and they,she or he may have given Sona away.It's evident Lee Sin is a held in high regard in Ionia and is an obvious target for assassins due to his protest of Noxian involvement and him setting himself alight.So Sona was given away to keep her safe.In a brief sentence it is said Lee Sin joined the monestary to atone for past mistakes.Maybe the mistake of leaving Sona's mother and her having to give up Sona.Or maybe him finding out Sona mom had died and tried to find what had become of his child.
Or Sona parents are just people Riot haven't decided to do anything with or about.
It is possible but a stretch,Ionia is home to Celestials and it possible she may be a half celestial or her parents are celestial forced to abandon their child or face consequences much like Soraka who was abandoned by the stars when she abused her powers to help others and lost her Divine form.
But if there will be an Ionia event which I hope happens.The obvious thing people would think would be Ionia vs Noxus.However I would prefer an Ionian civil war.Sona may be forced to leave Demacia and return to Ionia to discover her roots.Major lore characters would/could be Lee-Sin,General Irelia and Duchess Karma and the Kinkou Ninjas: Akali,Shen and Kennen,Bard,Soraka,Varus,Syndra,Zed,Yasuo,Ahri,Wukong and Master Yi.
But until then Sona is egnimatic and her true story will forever be a mystery.