Changing heal mechanics won't help Soraka
I just played against Soraka and had a thought: almost all her kit is about denying. Healing in MOBA game is only denying what enemy team is doing/has done. 4 of her abilities are centered about healing. Both parts of her passive are about making it harder to stop her from healing. Her only CC are silence and conditional root, they can be used to deny enemy abilities/movements, but it's really tricky to actually do something to harm enemies with that - it's not like they will get you killed. She does some damage, but it's tricky to get it to relevant levels.
Now compare it to another defensive support, Janna: She is mostly about protecting allies, but it is far easier for her to actually aggresive commitment to fights. She can set up kills with slow and knockup, she buffs damage, finally she can make some surprising "Janna insecs", even if that's rare.
I think that to become healthy champions again, Soraka needs some of her healing power traded for ability to actually do stuff.
). Now she has to hide in the back line and stack health regen, which is one of the things people call "low interaction" about her, or risk being easily executed after a few heals, which are larger now than they were before to make up for the fact that she can only heal for a short duration before going under the minimum hp level.