I'm also going to do one for Sona, but a hell of a lot more radical. I'm going to mostly be working with Powerchord, that's going to be the main focus of my rework, I'm also going to be looking at toning down her auras a little since they don't really work in practise.
For her passive I like the idea of "Invoker Sona". Rather than the drastically different effects Invoker has however the effects of Sona's Powerchord would be more self explanatory, adding Qs always adds damage, adding Es always adds movement impairing effects and adding Ws always adds damage impairing effects.
Passive:Powerchord
After Sona casts 3 spells her next auto attack or Q will apply power chord to her primary target. The effect of powerchord is based off of the last 3 spells Sona cast before Powerchord. Q won't count as a cast for a Powerchord it activates.
Base Effect
Damage:6+8/level(+30% AP) (150 base at level 18, down from 220)
Staccato
1 stack: Powerchord deals 40% bonus damage
2 stacks: Powerchord's effect and damage are applied in a 300 diameter AOE.
3 stacks: Powerchord increases it's own damage and the damage of the Q or AA as well as any other attached effects (ie. LB on an AA or Luden's on a Q) by 30%
Diminuendo
1 stack: Reduce enemy damage by 25% for 3 seconds
2 stacks: Reduce enemy damage by a further 25% and duration is increased to 4 seconds (50% total)
3 stacks: Disarm (Amumu's R effect without the snare) and Silence the enemy champion for 2.5 seconds, then reduce their damage by 50% for a further 2.5 seconds
Tempo
1 stack: Slows enemies by 40% for 2 seconds
2 stacks: Slows enemies by a further 20% and applies a ground effect for 2 seconds
3 stacks: Roots enemies for 2 seconds
What this allows Sona to do is chose the most effective Powerchords for her situation adding some extra mastery to the champ via decision making. You can go for Q2, E1 to get an AOE slow on the enemies or Q2, W1 to reduce damage in an AOE. W1, E2 cripples assassins with a damage reduction and a ground effect. The 3 stack abilities are all super powerful but require you to not cast other abilities while they stack, so it's a conscious tradeoff.
Q:Hymn of Valor
Sona shoots a skillshot in a straight line, upon hitting an enemy it explodes dealing damage in a small AOE and applying Powerchord to the primary target if it's avalible. Hymn of Valor will stack towards Powerchord on cast if Sona has two or less stacks but will not stack until after Powerchord application or the projectile fading if Sona has 3 stacks. Soma can store up to 2 uses of Hymn of Valor.
Range:950
AOE:300
Cost:45/50/55/60/65MANA
Cooldown: 10 seconds per stack, 1 between uses
Magic Damage: 80/115/150/185/220(+70% AP)
Aura Removed
Pretty basic ranged spell, Sona can use it to harass, control waves (which is actually an important element of supporting and a large part of the reason Sona doesn't really work) and apply her passive with some safety.
W:Aria of Perseverance
Heal up, CD down, no shield aura
CD:10>7
Heal:35/55/75/95/115(+25% AP)>40/70/100/130/160(+30%AP)
Aura removed
The CD on this is down to make it somewhat possible to stack Powerchords, the shield is basically removed to compensate for the increased spamability on Sona's W and increased Powerchord power with the heal up to make it not feel worthless.
E:Song of Celerity
CD Down
CD:12>7
Simple change to offset changes to R CDR and put the ability CD fairly in line with the rest of her kit. It should be about as easy to stack her spells as each other IMO.
R:Crescendo
Base ability CDR down
Base abilities now gain 10/15/20%
This might just look like a devastating nerf, but with the reduced CDs kit wide 40% bonus CDR would just be too much. The lower base CDR and lower CDR scaling should help give Sona a more noticeable amount of Early-Mid to Mid-Late game power that she's currently lacking.
Basically I'd like to see more thought about how Powerchord is used to add a little bit more mastery to the champ while still keeping its use cases straight forward to new players. You want to kill a guy, spam Q, you want to not be killed by a guy, spam W, you want to chase or not be chased by a guy, spam E.
Now that I think about it this is probably a bit larger scale than the Shyvana rework, it's more on par with things like the Ashe rework or Sona's update, but I just wanted to do it anyway.