I feel that she is still lacking team utility and is too heavily hampered by ignite and other healing reductions.
Basically she is a one trick pony with an obvious counter. Except now she is a one trick pony who gains the most healing when any sane person would have ignited.
Or in the case of the Morellonomicon, she gains the most potential at the exact moment the item screws her.
For a start, I feel like her passive could be made more powerful and less influenced by healing reduction if the effect was a shield rather than a heal. Shields are a much better clutch defenses since they can be larger, as they don't affect long term sustain.
Maybe something like:
"Whenever Soraka heals an ally with Astral Blessing or Wish, they gain a shield equal to X. This shield is increased by 1% for each % of missing health." with X scaling based on Soraka's level or mana or whatever.
The shield gets to scale on a 1:1 ratio since it is literally a clutch defense rather than permanent health and bypasses healing reduction.
As for Utility, I was considering Infuse. It currently offers a Silence when used as offense. This is good against mages, but what about melee? The armour is one thing, but she could do with something for a weaker ally, such as the ADC.
"...Infusing an ally with energy also causes the next melee strike to knock the enemy back and slow them by X% for 1.5 seconds."
This is a good defensive option versus some bruisers while not being as useful in the laning phase. It also means Soraka needs to choose between silencing abilities, or hampering movement.