My own personnal thoughts on Sona...
tl;dr: Gut Q, buff her auras
Now, before you start reaching for your pitchforks, hear me out.
Sona's Q is one of the most binary spells in the game, and is currently her entire kit. It isn't even point and click poke, it is auto-cast poke. We are talking about an ability nearly as cancerous as Vlad's Q or Nidalee's spears. Simply put, as long as Q is a strong ability, Sona will need to be weak in all other areas.
But, for me personally, that is fine. I do not like Sona's identity amounting to "the poke queen". She is an artist, a musician. She is graceful and delicate, that should be her identity.
How you may ask? I point you to before her rework. Her old auras, as invisible as they were, embodied this identity and fantasy. She decided the flow of a teamfight, buffing her allies with armor/MR or damage or speed. Her stun was the applause; the curtain call to finish her beautiful show.
Basically, what I'm suggesting, is a straight nerf to Sona's damage. But, to make up for it, bring back her old auras in some manner. Buff their range, make Q's aura not absolute garbage, buff the shield from W, that sort of thing. Make Sona the conductor, and the match will be her greatest performance. Her allies are the band, the enemy team is the audience.
Let her knock them dead.