A Look at Sona's Kit
At the moment, Sona's in an awful spot. Sure, her win rate is high but she has the most bland, unsatisfying kit in the game that serves no other purpose than to ult at the moment. Her poke is mediocre now, her heal is weak and costs a large chunk of mana, her speed up is hardly noticeable unless you have tons of AP, and her ult is just there: an aoe instant stun. Obviously, Riot has a hard time balancing this kit because it's so simple, that making it good only hurts Sona's health gameplay wise.
I'm bored, and have had this idea on my mind for a few weeks so I wanted to share it with everyone.
[Passive] Power Chord: AURA: Sona's basic abilities each generate a unique aura for 3 seconds that empowers her and all allied champions who enter its radius, also incurring a 0.5-second period during which she cannot cast her other basic abilities. Each aura's duration is extended by 0.5 seconds for each allied champion it empowers, up to a maximum of 5 seconds.
POWER CHORD: After casting 3 basic abilities, Sona's next basic attack deals「 13 - 192 」(+ 20% AP) bonus magic damage, with an additional effect depending on the last basic ability cast.
Power Chord resets Sona's autoattack timer.
This passive makes her unique, but also requires constant casting of her high-cost abilities to make the most use of it, which often means you wont get the most use of her abilities' passive... But really, that isn't so bad. I'd remove the aura duration, however, and just keep it permanent dependent on what stance she has active. Maybe have the auras linger for a second or two when switching to another stance.
[Q] Hymn of Valor Cost: 30 / 35 / 40 / 45 / 50 Mana Cooldown: 5 seconds
Active: Sona sends out a wave of sound that deals magic damage in a radius around her. This damage is split evenly among targets. Magic Damage: 60 / 120 / 180 / 240 / 300 (+ 65% AP)
Aura: Sona and tagged allied champions deal bonus magic damage on their next basic attack as long as they stay within the aura. Bonus Magic Damage: 20 / 30 / 40 / 50 / 60 (+ 20% AP) (+ 15 / 30 / 45 based on Crescendo Rank)
Power Chord Staccato: Enemy take 10% (+1% for every 100 ap) extra damage from all sources except true damage for 3 seconds
[W] Aria of Perseverance Cost: 40 / 50 / 60 / 70 / 80 Mana Cooldown: 5 seconds
Active: Sona shields herself and nearby allied champions for 3 seconds. Shield Strength: 50 / 75 / 100 / 125 / 150 (+ 25% AP) (+ 10 / 20 / 30 based on Crescendo Rank)
Aura: Sona and allied champions within the vicinity restore 1% (+1% for every 75 ap) of missing health every 0.5 seconds as long as they stay within the aura. ( Small radius so it's not abusive )
Power Chord Diminuendo: Power chord reduces its target's damage output by 20% (+ 1% per 100 AP) for 3 seconds.
[E] Song of Celerity Cost: 65 Mana Cooldown: 5
Active: Sona and allies gains bonus movement speed that decays to 50% after 0.75 seconds. 1.5 second duration. BONUS MOVEMENT SPEED: 10 / 12 / 14 / 16 / 18% (+ 5% per 100 AP) (+2% / 4.5% / 7% based on Crescendo Rank)
Aura: Sona and allies gain 15% (+1% for every 75 ap) attack speed, while enemies lose 15% (+1% for every 75 ap) attack speed.
Temp: Power chord slows its target by 40% (+ 4% per 100 AP) for 2 seconds.
[R] Crescendo Cost: 100 Mana Cooldown: 130 / 115 / 100
Passive: Sona's aura bonuses are increased.
Active: Sona plays an irresistible chord in a line, dealing magic damage to enemy champions stunning them for 1.5 seconds and, dependent on which stance is active, will cause an extra effect. Magic Damage: 150 / 250 / 350 (+ 50% AP)
Hymn of Valor: Enemy take 10% (+1% for every 100 ap) extra damage from all sources except true damage for 3 seconds Aria of Perseverance: Reduces target's damage output by 20% (+ 2% per 100 AP) for 3 seconds. Song of Celerity: Slows targets by 40% (+ 4% per 100 AP) for 3 seconds.
I don't know if this should be taken seriously or not, but I actually like the idea of Sona being this buff-tagging, debuff-tagging support that can spam her abilities to change the flow of a battle. Like a musician, controlling the battle with her rhythm. The idea is probably terrible, but it personally excites me to think of such a champion. Also her healing is very problematic to buff, which is why its in its current weak state. This way, instead, Sona can constantly shield her allies from incoming damage if used appropriately in its short duration and heal a miniscule amount of heal over time. I also like the idea that her stances have control over more than just her passive, but her ultimate as well. Riot might see this as too much invisible power, though, but it's so different and unique.
Anyway, I'm off to bed. Good night! 
TL;DR: Sona is a boring, bland champion whose kit is too straightforward but unrewarding.
