Let's talk about ARAM Sona.
Why? Well, she's currently sitting at a massive winrate. 70%. Next closest is 63% with Ziggs. http://www.lolking.net/charts?region=all&type=champion-winrate&range=weekly&map=ha&queue=normal
What's so strong about her? It's not just her ult or the heal, it's that she's able to just about everything. She can move in, hit you with a powercord + Q [adding on the lich bane and luden's procs] and walks away safely. She can sustain the team with a chalice and has rapid movespeed for herself and those around her. She can snowball ult and heal the team up with the mana regen that the map gives. The heal isn't a big heal but it's so spamable now. Her healing/sec is higher than before. She can speed up the team and slow one person down. She can also reduce their damage output.
Damage, sustain and utility. Soraka is close but the %hp and mana costs mean that Soraka needs to pick between tank/hp battery Raka or a damage mage Soraka. Sona can do it all with Chalice, Lich bane, Deathcap, voidstaff, ludens and boots. Or even sell the boots for another item.
Most of the balance is done for Rift, I get that. But we've had champions tweaked for ARAM. I think we might need to adjust that as well as the changes have really made her the ultimate carry/support that just does everything on the map. I think the issue is how fast her abilities come off CD. On rift she can't have the mana for it but on the abyss she rarely goes oom. Or perhaps increase the cost of her abilities as well?