A nobody's thoughts on how to improve Sona
Feel free to skip to the end for tl:dr
Ever since her rework, Sona has been considered weak. However, I'm not gonna say the rework was a bad idea. At the core, Sona's kit wasn't necessarily healthy. Press a button, give whole team free stats, press another, give other free stats, and the only things that made you feel useful was using Q + Passive combos in lane and landing good ults. The rework wasn't a bad thing as much as it didn't make the right changes. Sona no longer gives free stats, but apart from using the same combo in lane and landing good ults, she pretty much feels just as useless. More importantly, she is required to stay with her team to get the most benefits out of her kit, but her kit doesn't give her much in the way of actually doing so. You'll get butchered if you so much as try to keep with the frontline unless your team is so fed that the frontline, frankly, doesn'T need you at all.
The not-so-recent Soraka rework is, imo, a good example of a rework. Soraka is annoying : The sustain she provides is insane. She doesn't have taunt. She doesn't have a shield that absorbs incoming damage. She doesn't have an insane amount of damage. But you still have to focus her, because if you don't, unless you've got mad burst, her adc will win the fight. It's interesting because it forces you to focus the support through a different kind of mechanic. The only way to fight it is to counter it with burst. Champs like Annie, that can just go "go ahead, heal your carry, he'll die anyway" and if you can't do that, well you just gotta focus her because hey, that's how you'll win this fight. Do I like laning vs Soraka? Hell no. But I can admire the way she works nonetheless.
Sona, with some changes, could easily become the same way. Not with healing mind you, but force you to focus her through a gameplay pattern that makes it so focusing Sona before the adc (unless he gets caught). I also believe that it would make her much closer to her original design, a musician. Her previous aura system was because she would play various melody, and holding onto that melody would provide bonuses. Since her rework though, she's no longer playing a melody as much as playing a couple notes and then sitting on her ass waiting for cooldowns. Not gonna lie, my friend's little brother can do the same thing a flute, and no one calls him a musician.
What if her spells were channeled spells?
What if she could hold-on to a spell, kinda like how she'd hold-on to a certain melody? It certainly would make her more musician-like, as well as make her a rather unique champ (again, like soraka). Make Sona channel spells rather than casting them, giving it an initial mana cast and an other small one to keep channeling it.
Q : Instead of giving every character bonus damage on one AA. make it so as long as she keeps channeling the spell, the effect refreshes every 1 second (or 1.5, or 2, there's some tweaks to be made, but you get the point). And every 4-5 second, the initial damaging part of the spell gets resent to the two nearest champs or minions if no champs in range.
W : The rework made Sona's W weird, but it had a goal to it : Riot just didn't want to keep the basic heal. And they were right, a regular heal would be too similar to Soraka's. So they made the heal weaker and add a shield. At first this seems okay, then as you think into it gets dumb, until you finally realize that her heal was not made for "welp we lost that trade, better heal up". Sona's W is only good during a trade/fight. If you heal after a fight, the shield is wasted, if you shield before a fight to soak up poke, the heal is wasted. However, during a fight, both parts are good, you get to soak up some damage and heal some of it. But because heals are, to riot, toxic, it is purposely kept weak.
My point? Remove the heal. I've played Sona for long, the heal is cool, but never once have I picked Sona to heal. It's always been about the Q, passive and ult. So remove it. Make something less toxic out of it and make it more fun to play around with.
As she channels her W, the shield keeps refreshing a part of it. Riot reworked her W to make it less post-fight recovery and more in-fight mitigation. Take that concept all the way. Remove the heal, making it easier to balance as a shield, and then make that shield an interesting part of her kit. Do I want to keep the Q channel up, giving my ADC bonus damage every x second, or do I want to stop incoming damage. Say the initial shield is 60, and every second 1/3 of it refreshes. Stop focusing someone and the shield is back to full after 3 second as long as Sona stays near him.
And then do the same channel thing to her E, and keep the ult. A lot of this is going to need some tweaks, but most of you can probably see where I'm coming at. Where Braum forces you to work around his shield, by either baiting it out or splitting him from his ADC/team, or where you need to focus Soraka or bring big burst if you don't want her to keep healing up the ADC/team, the core idea here is to make you focus Sona through a certain gameplay pattern or work out ways to to counter her.
Any target(s) close to her will either get a lot of bonus damage coming from her chaneling her Q, or she can mitigate a lot of damage in a fight by standing by her team channeling her W. When chasing or even running, she can cast her E and give a constant speed buff to her team. Split her from her team however, and suddenly, her team gets weaker. By taking out Sona before a fighting, or zoning her completely, her team starts dealing less damage, starts taking more damage, and has less change to chase or run away.
This also opens up new ways to build her. The rework already made her less good with supports items, focusing pure AP over other stats to simply keep up with other supports in utility (which I'm not a fan of, personally). However with channeled spells, more items and paths become interesting. A tear item would work wonders, but what then? Get more AP to get more out of her her spells, or instead go down a tankier route, so that she can survive for longer during fights and thus, channel her spells for longer but also weaker than if she took the pure AP path.
And this doesn't necessarily kill her solo lane power, but also make it interesting in a way. In extended fights, if she channels her Q for a long time, she can put on a lot of damage simply through AA-ing and the re-application of her Q damage. However, if you force damage on her to the point of her switching to her W, she can mitigate a lot of sustained damage, but is still vulnerable to burst and most importantly loses access to the damage she'd usually get through keeping her Q up (who is now on CD anyway so she can't bring it back instantly). Also, the fact that she no longer has a heal means she can't sit back and heal like old Sona would do (or still does in ARAM and the occasional URF mode).
TL:DR
Make Sona able to channel spells, therefore being able to apply their effects multiple times through a fight. Remove the healing part of her shield and replace it with a better shield. Make her scale better with tankier items so she can keep her auras onlger during teamfights if focused, all while keeping AP a good stat on her. Finally keep her solo-laning viable by not butchering her ratios as much as just keeping counterplay to her channeling.
Not sure how this post is gonna turn, I don't usually make such posts. I only had a "sudden shower revelation" moment and had to type it out. Feel free to call me and idiot or discuss your thoughts about all this.
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--finished, of course, if the game permits me)...Side note on the W+ Passive reduced healing effects and the
and people will ragequit because Sona is ridiculous. (Note on the Lich Bane since it just occurred to me, I usually take that bot if I'm against Soraka, Janna, Lulu, Nami, etc. You know, them squishies supports. Makes Sona even more oppressive than she already is, trollololol...but leads to more kill steals. I regret nothing.)