Idea for how to make Sona's E really cool

So this kind of randomly popped into my head, but what are people's thoughts on adding a secondary effect to Sona's E? In this case i was thinking something like the ability to negate slows and roots, BUT only if the spell was active before the champion would be slowed/rooted. Given the very limited range of her auras and the long base cooldown, it makes her have to play very close to her ADC if she wants to get the effect to work. With long cooldowns and limited range, this would help separate the OK Sona's from the really great ones who can read their opponents and know when to use a other wise unimpressive skill.

This would also give Sona players something they can do in engages rather then just be focused around landing a good ultimate and/or getting a bunch of free Lich Bane empowered poke.