Making Sona's powerchord E snare could possibly be enough...
Like, she can't escape death in most situations outside of hitting Flash or ult. If she had a way to snare one dude for a bit after casting 3 spells, I think that's fair given her EXTREMELY squishy defenses and horrid move speed. Even Zyra has two forms of hard cc (they're both just about as squishy as each other), and nearly any other support has hard cc they can access in some fashion without burning a long cd ult. Sona's powerchord E should snare instead of throw on a "meh" slow that can be replaced by buying Rylai's and spamming Q. I mean, what else do people even use E for?

(and that's not always the case, if Janna catches her with a tornado and AD buffs the ADC, that could be trouble for Sona) 
is played for her early game dmg, sustain, and lane pressure. Her weakness is squishyness and lack of mobility.
While I do agree that her e is in a pretty weak spot atm, she doesn't need additional cc.