Sona Re-work/vert idea
So a lot of things are currently changing, and some reworks are about to be done, so with that it seems like an appropriate time to recommend some changes.
Sona's rework had some stated goals:
1. Make her kit more rewarding and fun
2. Improve her high moments
3. Increase engagement and interaction
4. Reduce time allies are effected by her abilities, creating exploitable moments of opportunity for enemies
These goals were not met, or not met in a reasonable way. A full explanation of why is here. So what this will be is a suggestion for a partial revert, and partial rework with the additional goals of bridging the gap between the kits, and adding some flat utility that puts Sona in a position where she's not considered a worse choice than non-supports with better flat utility (Shen, Poppy, Trundle, ect..)
Her passive would return to being 13 / 20 / 27 / 35 / 43 / 52 / 62 / 72 / 82 / 92 / 102 / 112 / 122 / 132 / 147 / 162 / 177 / 192 damage on power chord.
Starting with her Q
This would be the Revert part of things. The active portion would go back to 50 +50 per rank, and the aura stance would return giving 4 +4 per rank AD and AP over 1000 units.
Staccato would again double the damage of Power chord. This would reshift power, dropping her total teamfighting burst down slightly and returning the old aura to apply her aura effect over time. Lane Sona would still be able to apply large burst to a single target, while support Sona would not be penalized for building more support oriented items.
Moving on to the W
This would be more rework-ish. Armor, health, and Shields are functionally identical in effect, increasing the damage needed to reduce health to 0, while being different in implementation and counters. They also have different satisfaction and player feedback levels. For example, at level 18 base health, simply standing in Sona's old W aura was worth 50% more than a stack of feast for cho'gath, and that gap would increase if he had any health from items or even existing feast stacks. However due to implementation Feast, and it's corresponding health feels way more impacting than the armor/MR aura did.
So the idea is to change the W into a 1000 unit aura that increases maximum health by 3% +3% per rank. This health would function like Scarl health. Upon activating the stance the bar would fill with healing and regen, but would not fill instantly as to prevent abuse. As long as the champion is in the aura they have the health, but it exists on top of their normal health pool, to prevent situations like sona killing a low health ally by swapping auras. The Pool would be heal-able, but would be healed after the true health pool while taking damage before the true health pool.
The W active would remain a cuddle-zone, healing nearby allies. Starting with the current 35 +20 per rank, and seeing if it needs to be adjusted from there.
Diminuendo would be unchanged.
Now for her E
This will be the biggest change. Her Aura would revert to the old 1000 Unit Flat movement speed buff, but it would be increased to compensate for changes to the active, to 6 +6 per rank.
Her active E would be a cuddle zone that applies "Release" to effected allies. Released allies have any slows or immobilizes removed, and allies cannot be affected by them for 1.5s after being Released. This would only affect the slow or immobilize portion of a combined effect, so an exhausted champ would still do less damage and take more damage, they just wouldn't be slowed. This would be powerful flat utility that would allow Sona the ability to proactively attempt to save an ally. This would also make E an ability that may actually be taken at level 3 rather than level 8.
Tempo would remain unchanged
Crescendo
It would probably be appropriate to remove the Passive CDR for her abilities.
I get that Riot is unwilling to go back to the old kit, but maybe her 3rd rework could reintroduce interesting and unique aspects of the old kit that were removed, while actually fixing some of the issues that they wanted to address. Right now when Jana gets nerfed, Sona's on top of the food chain again, and that never really ends well for her, due to the unbalanceable nature of her kit.