Sona's story is kinda... unsubstantial when you get down to it. It just describes her as a super-busty musician with a strange instrument, and not much else.
How do you think she should have her story expanded?
Honestly I dunno. I don't know how Sona is even supposed to have a developed story. I mean it's implied that she and Xin are on friendly terms because of a linked Ionian origin, but that only takes you so far. So let's boil things down: What is Sona's position in the world?
I feel like I'd like to see Sona's role in Demacian society explored. What the hell does she even do? Is she like a rich person who lounges around not getting involved with petty affairs, or is she involved in Demacian high society? What is Demacian high society even like? Does her position allow her to get involved with politics? What champions and other important Demacians does she associate with?
I think the main thing that Sona has going for her in terms of greater story involvement is her role as a part of the nobility. Her personal stories will always be tied in with music and the Etwahl, but I personally want to see more about her place in the world and how the world treats her now that she's no longer a player on the world stage that was the IoW. She almost feels like a secondary character to me because of how disconnected she is to other champions within Demacia, but I also feel like that's something that will depend entirely on how she's written.
Hopefully that made some modicum of sense to somebody.
Maybe it's demon possessed?
I'ma link demons to so many things now that I know their a thing. Pallas? Zed's shadows?? Nocturne??? DEMONS!!!
On a serious note though potentially the Etwahl is an object possessed by a demon(s) and it feeds off of the energies produced by people who hear it played. It's potentially not a dangerous or even hostile entity, it's just one that's trying to survive and found a more creative way to do it, with the magic coming from it being as a result of the demon. I dunno just spitballing here.
Calling it, the Etwahl/Sona is secretly a Darkin.
More connections to other champions. a real reason to be fighting
Appear in a game where the company cares about the story
Honestly I think her story should focus on her etwahl and what the heck it really is. I mean she has this music instrument that gives her magical powers and that is following her around all day and no one knows where it came from?
The etwahl is cursed, mark my words.
fk tht, giv it to 
The etwahl is zed from the future!
For starters, change her VO for something else.
