Soraka has some problems. I've pointed them out elsewhere, but for those that don't want to dig up old posts...
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She's actively penalized by building health (a stat found on a LOT of support items and essential to countering pure damage as well as boosting EH), to the point where if she levels the wrong ability first (hint: anything but W) while buying normal support items it can result in a net team health loss every time she casts her heal.
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Her mage secondary is completely nonviable, because she's A. squishy, B. lacks mobility, C. lacks reliability on her damage and D. has weak AP scaling to go with it all.
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She only really benefits from a narrow range of items. AP isn't amazing (it's unnecessary in lane and useless later on), health actively makes her worse, she only needs one mana regen item. That leaves a fairly narrow set of support utility items and the few tank items that don't have tons of health on them.
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She has a very aggressive kit for a very defensive champion. She's rewarded for being on the front line (with a more accurate Q, a short range on her W, and to better position her E) but is heavily penalized for being caught on said frontline due to lacking mobility and rarely (if ever) building HP.
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Her E is insanely overpowered in teamfights, to the point where playing Soraka consists of "healbot with W in lane; press E before dying in teamfights."
Now, make no mistake, her kit is far better now than it was pre-rework in terms of health-of-game and balance. It just has some pretty big issues still to take care of.
If you want my opinion, she should have two different build paths: One focused around support and healing that uses health and tank items, and one focused around her mage secondary that focuses on AP and damage. The changes would look something like this:
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Her passive would increase the effect of regeneration and healing gained from her Q by (100 * (bonus health / base health))%. Essentially, if getting a regeneration item caused her to gain 1% of her health per 5 seconds at level 1, it would continue to give 1% of her health per 5 seconds even if she bought an item that increased her maximum health pool.
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Her Q would have a lower baseline damage, higher AP scalar, and shorter cooldown. Its healing would scale with Soraka's bonus health instead of AP.
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Her W would would sacrifice 10% of her maximum health to heal an allied champion for X + 10% of Soraka's maximum health, with X being a constant depending on spell rank.
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Her E would remain basically unchanged.
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Her R would, instead of being a constant plus an AP scalar, would simply grant a percentage of her maximum health to everyone on the map.
I'd see her as a viable choice for support, top lane, or mid lane.