Soraka is imo currently crazy strong and weak at the same time. Laning phase she is godly. She must partner but that lane is borderline unkillable early game. Also, with a proper laning, like Lich, she can really let them farm to hell and back with infinite mana and their low CD nukes. This means that the game can technically end very fast.
Her problem however is that she scales to end game in a piss poor way. Nobody really need her mana that bad after a while, and her general heal turns to **** also. Her ult is the only useful thing as far as I see it after a point. And at the same time while you had massively stronger power early from her spells, you are now underpowered due to her not being able to do anything except try with her heal and wait for her ult heal to cooldown.
Another burst heal would help her a bunch I think be because the HoT just doesnt cut it with current late game damage. If anything the armor needs to scale with AP as well as the heal. If you could boost your allies armor by like 50 that would mean another 5-10% of reduction in most cases, which is significant.
id also like to point out that Soraka probably gains the most of her strength from getting the Locket item. This item basically outperforms her built in heal but it stacks with it so its nice. But Im pretty sure she cant increase her heal capability anywhere near the power of that Locket alone with just AP based items, which is weird.