Can't post about Sona at PBE boards, I'll ask here, why not buff passive instead? I'll explain why

In my opinion the lack of CC along with her squishiness it's her main problem. It's a conjuncture of both along her being a support. Instead of changing her movement speed on her E, why not increase her Power Chord strength? Buffing her base values won't fix her problem of being viable on the meta and also it can make her scary powered, which can force again a nerf (how many times we've seen buff then nerfs followed?). She needs more utility in order to be present, specially in her E. Making her passive stronger along with E would make players feel present and as well make E less underwhelming, which I explain below.

Q Power Chord would been better if it decreased the armor of the enemy to encourage trades with your ad carry, even if that meant you having less damage yourself on the Q Power Chord passive, which goes along with Riot thinking of being scared due her being able to nearly one shot a squishy late game.

W Power Chord is fine, I just would prefer a better heal itself on the W spell. A minimal buff is enough, nothing too radical. If not, then decrease it's mana cost. 100 mana at late game is way too much for a heal of that strength.

E Power Chord should have a bigger slow. Let's be real, the power chord is situational, it forces to use 3 spells in order to use it properly (which means you can run out of mana fast) and on a teamfight only a good Sona player is capable to use all 3 spells properly to use it on the fight AND to use passive well. Increase the slow of power chord on later levels.

R Is fine, I would just decrease the cooldown a bit, since Sona's CC is very dependent on the ultimate it should have lesser CD, even if that means lesser width on the spell.

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TL;DR:

• Increase Power Chord** Utility** strength, even if that means a little decrease on the damage. Q Power Chord should decrease targets armor to encourage trades with AD carry. Q Power Chord can deal less damage itself since the decrease or armor itself it's a pretty good utility buff;
• W spell needs a better heal, period. The shield aura is already strong since it's AoE;
• E Power Chord should have a better slow to let her being able to set up ganks besides her ultimate.
• Buff her utility, not her raw base power. She already deals enough damage and shield is fine. Fixing her raw base values won't change her state because she still won't fit the meta. Make her useful, not overwhelming.