As a person who mained Sona back in Season 3 and only ditched her after the nerfs they threw on her post-rework I think I can say a few problems with Sona (Keep in mind I'll be comparing her to traditional supports, not ones which were shoehorned into support (Sorry Karma, Zyra, Annie and Morg)
-
How you play Sona now doesn't match up to her stats
Pre-rework Sona would switch Auras depending on the situation, she was essentially a buff bot occasionally dipping in when she has 3 stacks and poking this is why back in the day Cait/Sona was such a strong combo, Sona was about staying back giving the ADC stats while occasionally poking but still having means to escape, this was balenced out by her stats making it so if you so much as looked at her weird she would explode, The rework turned her into a mid/close range cast spammer, yet her stats didn't change, her mana costs went up and basically made her the most risky support in the game, for little reward mind you.
-
The other supports/new supports now do an assortment of her jobs but better
Damaging poke?

Heal and shield?
(literally)
Movement speed?
Engage?
Peel/Protecting VIPs?
(Don't mind me ima hang back and use ALL THE HEALS)
(Nom)
Buffing allies?

So a large amount of supports do her job but better, Hell Nami does ALL HER JOBS better, if you wanna play a champion like Sona you don't pick Sona, you pick Nami. Theres a saying "Jack of all trades and master of none" well Nami sidesteps the last half and Sona smashes right into it
Shut down a Bard? he still has his E and his ult, Braum? Shield? Thresh? Screw you he's Thresh, Janna? She justs shields damage dealers, Sona? Well now she'll just have her ult which given she hasn't afforded T2 boots yet and her flash is down means we won't be seeing it before we blow her up again.
Sona's base damage is low, really low, low enough that outside of the first 4 levels unless she gets some AP going she'll be dealing no damage to anyone, this again goes to point 1, she has the stats of a hang back support but was changed to an aggressive poke support without any of the stats changing where as any other support even shut down has something in their kit to ether help them hang on or just make then say "Who needs items?"
Ah Mobility creep, a problem that Riot achknowledged but refused to fix, how it shows up now
Sona has 325 move speed, lowest base you can get the only other support that has that is Soraka, y'know the support who's job it is is to just hang back and heal? yeah that one, then we get to the aggresive supports like Alistar and Leona, oh hey 330 for Ali 335 for Leo, and I bet they're packing movespeed runes as well.
Again points to the whole changing her playstyle to another one without changing the stats
At the end of the day the real problem with Sona is:
Her playstyle and stats don't match
She is now outshone by every support (and Nami is Sona 2.0)
no level of buffing will solve the problem with Sona, she needs one of 2 things
- Revert her rework
Ok lets be real here, Riot doesn't like hidden power but when they reworked Darius they kept his hidden power of his Armour pen, Zed still has his Additional AD per point via his W, Kindreds passive is essentially hidden power, Azirs W IS HIDDEN POWE- Notice summat here? Riot CLEARLY don't hate hidden power to the point where they stop giving it, Sure Sona might not of been the healthiest of supports back in S3 but we're about to hit S6, stuffs changed Sona could fit in nicly if she was Pre-rework
Unfortunatly Riot would rather panic attack and swing the buff/nerf hammer around then EVER revert anything pushed to live, it went to live so it was and always will be the right choice is Riots logic I guess
-
Re-Rework her
I nominate CertainlyT for it, while I hate the guys champions the uniqueness and (unfortunately) Overtuned overpowered kits he gives to champions would mean Sona would (should) be able to compete with the recent supports and i some cases win (Honestly I'd love to see how he'd make Sona without every ability being an engage tool
)
-
NERFS FOR EVERYONE
No
Lets not kid ourselves here though ANY change Riot makes it's very doubtful they will care much about her identity, they haven't in the past. We'd just have to choose between keeping her incredibly weak and keep her identity or bring her back and HOPE her identity hasn't been changed too much.