Malfurion's latest kit in Heroes of the Storm is a good example of how to make a good "healer mage", IMO. The way it works is as follows.
>**Q - Regrowth**
Heal target ally over 20 seconds. 5 second cooldown. Does not stack.
>**W - Moonfire**
Deal weak magic damage in a small area and reveal enemies struck a short time. Allies with an active Regrowth are healed for X health for each enemy Hero hit by Moonfire.
By itself, Regrowth will not heal very effectively due to spreading its healing over such a long duration, but when you apply it to multiple allies and then start hitting enemies with Moonfire it becomes very effective.
Soraka presently tries to do an inverse of this with her Rejuvenation effect along with sacrificing her health to heal allies, but this constantly leaves her in "balance sensitive" situations where a slight buff can make her broken. I think that if she were reworked (AGAIN) to use a style similar to Malfurion, it could promote some much healthier gameplay, requiring you to be aggressive to get real mileage out of your healing.