This concept completely revolves around the idea of the community which is to shift her into a "spell-order" based mage, similar to the gameplay of Invoker from DotA. Though I've personally never played DotA before, I'm doing my best to create an idea that doesn't stray too far from what it's supposed to be.
In this concept, she might become more of a jack-of-all-trades champion, due to her songs having variety (ten songs?).
Circle of Melody will allow Sona to retain her identity as an aura support, and is similar to her old gameplay, only that it rotates without the constant use of her abilities. To play around this, Sona and her allies will have to time their engages and trades to have an advantage in the situation, like engaging in a trade as soon as the first string aura comes up, and escaping and reducing damage taken at the same time as soon as the second string aura comes up.
Also, to hinder too much fast spamming, and to stop her from changing rotations every now and then, she will have a shared cooldown on her basic abilities when casting the maximum amount in a short period of time. This concept is made for fun, so some things can be random and far-fetched. I also didn't bother to give numbers because I'm not that familiar with them. Anyways, I will freely accept any kind of feedback, and will change whatever strays too far from the concept or what doesn't fit her. Thanks!
Her ult will be having two versions, depending on which is best, where one is tiered by effectiveness and the other revolving only around 3 strums per song. This was suggested by Frankenheimer.
Here are the songs' list by tier:
Tier I songs are all linear pass-through skillshots, and need 3 ordered strums to play. They are playable at all levels.
Tier II songs are all cone skillshots, and need 4 ordered strums to play. They are playable when her ultimate is at level 2.
Tier III songs are all small AoE abilities, channel for 2 seconds before cast, and require 5 ordered strums to play. They are playable when her ultimate is at level 3.
Tier IV songs are all AoE, channeling abilities, and require 6 ordered strums to play. They are playable when her ultimate is at level 4.