Sona would thematically fit a Invoker skill set.

She does music and music is creativity. You can make all kinds of notes and play tunes with music. Sona should reflect that. Q,W,E becomes components to create a debuff/buff. The execution is through right clicking. Combinations, made of 3 components, from Q,W,E. Sona could have over 7 abilities and effects when Q,W and E are combined in different ways. Her ultimate will still be there as her R.

She'll be the hardest champion though.

I'll probably get downvoted for trying to copy DOTA and Dota 2 but I think it would be interesting.